Legacy software modernization without taking the system offline
Somewhere in your company is a system nobody dares touch. FileMaker, Access, a Visual Basic program, a PHP app from 2009, a vendor who stopped answering. It still runs the orders. So the rebuild scares you more than the old system does, and that instinct is correct. Deepsel's legacy software modernization starts with a $3,000 assessment week that ends in a written map of the system and a staged plan, and then we replace it one piece at a time behind the running system, on published engineer rates, with a way back at every step. The Swiss federal administration's central login was rebuilt this way, live, with zero downtime.
The system your business runs on, and why nobody touches it
The companies that land here have 2 to 300 people and a database or program that one person built, often over 15 or 25 years, with the business grown around it. Four things send an owner or office manager looking for help.
The person who built it is gone
Retired, hired away, moved, or died, and the passwords went with them. Twenty years of how the business works lives in that database, and nobody left can open the hood.
A Windows, Office or licence change broke it
Reports stopped previewing after an update, or the vendor's support date passed, or the renewal doubled. The system still works, on one machine nobody dares restart.
Customers ask for things it cannot do
A portal, a link to QuickBooks or the web shop, a second location on the same system. Meanwhile the admin prints the orders and types them in again.
The vendor holds the keys
Every change is a quote, every quote grows, and read access to your own data is an extra line on the invoice. Leaving feels impossible because they have the only copy.
We don't call it legacy to your face. That system holds rules nobody wrote down: how a rush order is priced, which customers get net-60 terms, what happens when a lot is split between two trucks. We treat it as the specification, and the first thing we produce is a map of what it does. One owner on r/MSAccess said what many think about letting anyone in: "The database works perfectly. Maybe I have fears about a real expert looking behind the curtain." Our job behind the curtain is to write down what it does, not to grade it.
What legacy software modernization means, and the six ways to do it
Legacy software modernization means changing an old system so it keeps doing its job on technology people can still hire for, host and secure. The industry sorts the options into a list of R words; AWS's version has seven. Each one is in the table in plain English, with the durations one vendor, Catio, publishes for each (its own table, October 2026), and which ones we do.
| Approach | What it means | When it fits | Duration (Catio's table) | At Deepsel |
|---|---|---|---|---|
| Keep and maintain | Leave it as it is. Fix what breaks, keep the backups honest, add the odd feature. | It works, nobody needs much new, and a developer for the stack can be found. | Ongoing | Yes, on the monthly engineer, when the assessment says so. |
| Rehost | Move it, unchanged, to a new server or the cloud. | The hardware is dying and the application is fine. | 1 to 3 months | Yes, often as step one of a staged plan. |
| Replatform | Same screens, new database or runtime underneath. An Access front end on SQL Server, for example. | The data layer is the weak point: size, corruption, multi-user limits. | 2 to 6 months | Yes. |
| Refactor | Rewrite the inside, keep what it does and how it looks. | The code is the problem and the features are right. | 3 to 18 months | Sometimes, usually folded into a staged rebuild. |
| Rebuild in stages | A new system, built one piece at a time beside the old one. Each piece is switched over when proven; the old system stays until the last piece. | The system has to change shape and the business cannot stop. | 9 to 24 months for a full rebuild in Catio's table; a staged plan ships its first piece in weeks | Our default. How the Swiss federal login was done. |
| Replace with packaged software | Buy an ERP or CRM and change the process to fit it. | Your process is standard and the odd rules in the old system were never worth keeping. | Depends on the package | We say so when it is true, and walk away. |
Those durations are one vendor's estimate for enterprise systems, and they're here because no other page we found on the subject prints any. A 20-user Access system is smaller than anything those numbers describe. The assessment week replaces the table with a plan for yours.
The assessment week, day by day, and what you hold at the end of each
Every modernization at Deepsel starts with the same five days, at a fixed $3,000. Each day ends with a document in your hands, so you can judge the week without reading code.
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Day 1: inventory
Where it runs, what talks to it (QuickBooks, the web shop, the card terminal, the spreadsheets around it), where the backups are and whether one restores, who holds each password and licence. You hold the inventory by the end of the day.
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Day 2: the map
We sit with the people who use it and write down what it does: screens, tables, reports, and the rules nobody wrote down. You hold a map of the system in plain words, the document the last developer never left you.
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Day 3: a safe copy
A copy of the system you can test changes on, so nothing is tried on the live one. Then the dangerous things closed: a backup that restores, an exposed credential, a server nobody has patched.
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Day 4: critical fixes
The faults that could stop the business this month, fixed on the copy, tested, then applied to the live system outside working hours with the old version kept ready. You hold a change log in plain English.
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Day 5: the plan
A scorecard on the system's health and a staged plan: which piece moves first, how long each takes, what it costs on our published rates, and an honest answer to whether it should be rebuilt at all. Yours to use with Deepsel or anyone.
Then the staged rebuild: how one piece moves
If the plan says rebuild, the work runs on the monthly engineer rates and every piece goes through the same four steps. The old system stays in use until the last piece is proven.
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Pick the piece
The one that hurts most or stands most alone: the reports, the customer portal, the order screen. Small enough to ship in weeks.
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Build it beside the old one
The new piece reads the same data the old system does, so both are right at once. Built with the people who will use it. You see a demo every two weeks.
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Switch it over, with a way back
Outside working hours, with the old screen still there. If anything is wrong on Monday morning, staff use the old screen and we fix the new one.
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Retire the old piece
When nobody has opened the old screen for a month, it goes. The data, the documentation and the code sit in your accounts the whole time.
Legacy software modernization cost at Deepsel, with the arithmetic shown
We checked the ten pages that rank for "legacy application modernization services", and none prints a price. Ours is the assessment at a fixed fee, then the rebuild at a published monthly rate per engineer.
Rescue Sprint
$3,000 fixed, one week
The assessment week above. One week, one time, for a system that runs the business but nobody can explain.
- Full audit of product, code and architecture
- Critical bugs and security holes fixed and shipped
- Plain-English scorecard and roadmap, yours to act on with Deepsel or anyone
Step one for every modernization. It is the same $3,000 sprint listed on our fractional CTO and project rescue pages.
Senior Product Engineer
$4,000 / month half-day, or $8,000 / month full-time
One dedicated senior engineer, a demo every two weeks, a daily overlap window with US hours.
The staged rebuild runs on this rate, one or more engineers, month-to-month. Also the right plan for keeping a system you decide not to rebuild.
Technical Leadership
$15,000 / month
A fractional CTO who takes charge of your technology and answers for it personally, plus the senior engineers who design, build and ship.
- Architecture, roadmap and product decisions
- Senior engineers who build and demo every two weeks
- Investor and tech-diligence support
- Monthly plain-English scorecard
For a company whose old system is the product it sells, with nobody on staff to own the technical decisions.
The arithmetic
One full-time senior engineer for three months is $24,000. Six months is $48,000. A year is $96,000, and two engineers for six months is also $96,000. A half-day engineer halves each of those. The assessment week tells you which of these your system is, piece by piece, and because the retainer is month-to-month you can stop at any month's end with the finished pieces running in your accounts. There is no hourly meter and no overage. One FileMaker owner on Reddit agreed $5,600 of changes and got a bill for $10,600 a month later without warning (November 2025); a flat monthly fee exists to make that impossible, and our guide to cost management in software outsourcing covers the other ways a bill grows.
What the market charges
Vendor price lists: ScienceSoft prices a restyle or rehost of a small app at $10,000 to $15,000 and a deeper modernization of a mid-size or large app at $50,000 to $250,000; Digiratina says small applications cost about $20,000 and take 3 to 12 months (both fetched October 2026). First-hand figures from buyers on Clutch: a 15-year-old customer portal rebuilt for about $140,000 between October 2023 and July 2024, and 35-year-old software rebuilt for about $285,000 between August 2023 and March 2024. At the enterprise end, a vendor executive in Forbes (February 2026) describes $1 million to $3 million over 12 to 18 months. Our custom software development cost guide covers the wider ranges, and if the old system is a product you sell, SaaS development cost is the closer read.
Guarantees on every plan
- Month-to-month, 30 days' notice, no termination fees.
- First 30 days: full money-back guarantee.
- IP yours from the first commit under a US-jurisdiction agreement; repos and servers in your own accounts.
- Any engineer replaced within 7 days if not a fit.
- Happiness Index: you score us every month on delivery, accepted work and incidents. A month under 50 is refunded.
A live government login, rebuilt in stages with zero downtime
eIAM, the Swiss federal administration
eIAM is the central login of the Swiss federal administration. It authenticates federal staff, cantonal partners and companies filing electronically, so taking it offline was never an option. We replaced years of patched legacy templates with a responsive interface built to the federal design system. The new front end plugs into the existing backend exactly where the old one did, so the backend stayed untouched and the rollout happened in stages with no downtime.
- Full front-end replacement on a live system, rolled out in stages
- Swiss federal design system and WCAG AA accessibility, verified in acceptance testing
- Tested on phones, older browsers and locked-down government workstations
Who does the work
I'm Tim Tran, Deepsel's founder, and I lead every engagement myself. Khuong Pham (7+ years, full-stack React and Python) builds, and Trang Hoang (QA, 6 years of test automation) checks each piece against the old screen before it switches over. Three senior people, a short client list, a daily overlap window with US hours from Hanoi, under a Delaware corporation.
We also run Clubloungery, a nightlife booking marketplace that takes bookings and payments, in production, so the standard of backups, monitoring and documentation we hold your system to is the one we live with. And if what you have is a half-built app rather than a 20-year-old system, the same week runs as a software project rescue.
Why legacy system modernization projects fail, and what we do about each
You fear the rebuild more than the old system because you've heard the stories. Four ways a rebuild goes wrong, with the numbers where they exist, their sources and limits, and what we do about each.
The big switchover
The classic failure is a replacement built in secret for a year and switched on over a weekend. A survey commissioned by the vendor Advanced found 74% of large enterprises had started a modernization and not finished it (2020, companies over $1 billion). A vendor executive writing in Forbes (February 2026) puts the traditional programme at $1 million to $3 million and 12 to 18 months, and says it fails between half and three quarters of the time. Our answer is the staged rebuild: nothing is switched on that cannot be switched back, and the old system stays until each piece has earned its place.
Staff go back to paper
Long-serving staff are fast on the old screens, and a new system that takes more clicks loses to a notepad. So each piece is built with the people who use it, measured against the old screen, and has to be faster before it replaces it. The person who knows why one customer's invoices still go out on paper is on the design calls.
Dependence on one person, again
Most of these systems had one developer for 5 to 25 years, and that worked until it didn't. The code, the data, the documentation and every password sit in your accounts from the first week under a US-jurisdiction agreement. Three named senior people know the system, and the retainer is month-to-month, so staying with us is a choice you make again every month rather than a lock.
Maintenance eats the budget
US federal agencies report spending about 80% of more than $100 billion a year on operating and maintaining existing IT (GAO, July 2025). CIOs at large companies told McKinsey that 10% to 20% of the budget meant for new products goes to tech debt instead (McKinsey, 2020, 50 CIOs). The "60 to 80% of IT budgets" line on most modernization pages traces to none of these, and we could not find a primary source for it, so we don't use it. Our answer is the arithmetic in the pricing section: a staged plan has a monthly price, and you can stop it.
That owner's first need was a backup that restores and a second machine the system runs on. Both are day three of the assessment week, before anyone talks about rebuilding anything.
Other ways to deal with an old system, and when each is the better buy
Keep it, with a developer who knows the stack
The right call when the system works and nobody needs much new. FileMaker and Access specialists exist; owners on Reddit accept around $215 an hour for them (November 2025). The risk is the one you already lived through, so ask for documentation, your own copy of everything and notice before any overage (our guide to risk management in software outsourcing has the longer list). Whoever you hire next, the assessment week produces the first two.
Packaged software
An ERP or CRM is cheaper than custom when your process is standard. The anchors owners quote: a $90,000 NetSuite proposal before implementation (March 2026), and a $40,000 inventory system the staff refused and the owner switched off (February 2026). Our guide on how to build an ERP system covers when to buy instead.
A large software modernization company
Right for a bank with a mainframe. ScienceSoft lists $50,000 to $250,000 for a deeper modernization of a mid-size or large application and $400,000 to $2 million company-wide (its own price list, October 2026). That is the wrong size for a 20-user Access system, which also gets the least attention at a firm whose other clients are banks.
Rebuild it yourself with AI tools
Some owners do. One operations lead spent AU$36,000 over 12 weeks including offshore help and penetration testing (August 2026); another scrapped a first version when it slowed down under the real data. If you go this way, the assessment week works as a review of what you built before the business moves onto it. Our AI MVP development page covers how we build with the same tools.
If the old system is the product your company sells, the rebuild is SaaS development with live customers on the old version, and if nobody on staff owns the technical decisions it needs an owner as well as engineers: that is the fractional CTO retainer, CTO as a service if you know it by that name, or an interim CTO if the person who left held that title. Our guide to choosing a software development company lists the questions to ask any software modernization company, us included.
Legacy application modernization questions
What is legacy application modernization?
Making an old system run on technology you can still hire for, host and secure, without losing what it does. That ranges from moving it untouched onto a new server, through swapping the database underneath it, to rebuilding it piece by piece; the table on this page lists all six approaches in plain English. For a small or mid-size company the system is usually FileMaker, Microsoft Access, a Visual Basic program, an old PHP or ColdFusion web app, or a vendor product the vendor stopped improving.
How much does legacy software modernization cost?
At Deepsel, $3,000 for the assessment week, then the staged rebuild on published rates: $4,000 a month for a half-day senior engineer, $8,000 full-time. Three months of one full-time engineer is $24,000; six months is $48,000; two engineers for six months is $96,000. The assessment says which of those your system is. For comparison, ScienceSoft lists $10,000 to $15,000 for a small rehost and $50,000 to $250,000 for a deeper modernization of a mid-size app (vendor prices), and two first-hand Clutch reviews report about $140,000 and $285,000 for single-application rebuilds that took seven to nine months. Our custom software development cost guide has the wider ranges.
How long does legacy system modernization take?
The assessment is one week. After that, the first piece of a staged rebuild ships in weeks and the whole plan runs for as many months as the system has pieces. One vendor's table puts a rehost at 1 to 3 months, a replatform at 2 to 6 and a full rebuild at 9 to 24; the two first-hand Clutch rebuilds took seven and nine months. The staged plan on day five gives you the number for your system, piece by piece, and you can stop at any month's end with the finished pieces in your accounts.
Will the business go down during the switch?
No single switch exists in a staged plan. Each piece is built beside the old one, switched over outside working hours with the old screen still available, and retired only after nobody has used the old one for a month. The Swiss federal administration's central login was replaced this way while it stayed live, with zero downtime during the switchover.
Do you work on FileMaker, Microsoft Access, VB6, ColdFusion or an old PHP app?
Yes. We read the old system to write the new one; the map on day two is the specification. The new pieces are built on a mainstream stack that any future developer can pick up (React or Vue, Node.js or Python, PostgreSQL), hosted in your own cloud account. When the right answer is to keep the old system and maintain it, the assessment says that instead.
Will my staff have to learn a new way of working?
Only where the new screen is faster for them than the old one, because otherwise it does not replace it. Your staff sit on the design calls, each piece is timed against the old screen, and the old screen stays until they've stopped opening it. More clicks than before sends people back to paper, and that counts as a failure on our scorecard.
What happens if Deepsel disappears, like the last developer?
You lose nothing you need. The code, the data, the documentation and every credential sit in your own accounts from the first week, under a US-jurisdiction agreement with Deepsel Inc., a Delaware company. Three named senior people know the system. The retainer is month-to-month, so the arrangement runs on your choice each month, and the written map from the assessment is enough for any competent developer to carry on.
Should we buy an ERP instead of rebuilding?
If two packaged tools can do your process without workarounds, buy one. When both fail at the same specific thing (your pricing rules, your lot tracking, your scheduling), that thing is what a custom rebuild is for, and the rest should stay standard. The assessment week ends with that answer in writing, and we say so when buying is the honest call. Our guide on how to build an ERP system walks through the same decision.
Where is the team, and who does the work?
Deepsel Inc. is a Delaware company. I'm Tim Tran, the founder, and I lead every engagement myself with a senior team in Hanoi, Vietnam, on a daily overlap window with US hours. Calls, demos and documents are in English, and the people on the first call are the people in the code.
For something new built beside the old system, see MVP development, MVP development cost, SaaS development, software development for startups and our list of small software development companies for startups.