Services · Legacy modernization
Legacy software modernization services that replace the old system one module at a time
We modernize applications that still run the business but can no longer be changed safely: .NET Framework, PHP, AngularJS and old Java systems moved to current stacks in slices, with the old and new running side by side until the last module is retired.
Short answer
Legacy software modernization replaces or restructures an ageing system without stopping the business that depends on it. Innovation Insight starts with a $5k to $15k assessment that produces a module map, risk list and migration plan, then migrates module by module using the strangler fig pattern, with a test harness around the old behaviour and reconciled data. Modernization projects run $40k to $150k+ over four to twelve months, or a dedicated team from $4,000 a month per engineer.
Reviewed by Zain Khalid Malik, CTO & Co-founder · Updated
When to modernize, rewrite or retire
Old software is not a problem by itself. A ten-year-old system that is stable, patched and cheap to change should be left alone. The case for modernization appears when the cost of not changing starts to grow: a framework out of support, a release that takes a week of manual testing, a database nobody dares to migrate, or the one engineer who understands the code handing in notice. The first job is to decide which of three paths fits, and it is often different for different parts of the same system.
| Signal | Usual answer | Why |
|---|---|---|
| Runtime or framework out of vendor support (for example .NET Framework 4.6.2 ends support in January 2027; PHP 7 and AngularJS are already unsupported) | Modernize | Security patches stop; compliance and insurance questions follow |
| Business logic is sound but deployment and scaling are painful | Replatform first | Containers and managed databases fix most of the pain without touching the code |
| Changes take weeks because everything is coupled | Incremental modernization | Pull the modules that change most into new services; leave the stable core for later |
| The product itself is wrong, not just the code | Rebuild with new scope | A faithful rewrite of a bad product is wasted money |
| Fewer than a handful of users, or a SaaS product now does the job | Replace or retire | Buying beats building when the function is not a differentiator |
Six ways to modernize a legacy application
AWS describes seven migration strategies; the six below are the ones that change the software rather than just its location. Most projects combine two or three.
| Option | What it means | When it fits | Relative cost |
|---|---|---|---|
| Rehost | Move the application as-is to new servers or the cloud | Hardware end of life, data centre exit, no appetite for code change yet | Low |
| Replatform | Keep the code, change what runs it: containers, managed SQL, a current OS or runtime | Deployment and scaling pain; a quick security win | Low to medium |
| Refactor | Improve the existing code in place: tests, dependency upgrades, module boundaries | Code is reasonable but neglected; the language is still viable | Medium |
| Rearchitect | Split a monolith into services or modules and introduce queues, APIs and a new front end | The system must change often and in parallel by several teams | Medium to high |
| Rebuild | Write the application again on a modern stack, migrating data and behaviour slice by slice | The language or framework is dead, or the code is beyond repair | High |
| Replace | Retire the system in favour of a commercial or SaaS product plus integrations | The function is generic (HR, billing, CRM) and not a differentiator | Low to medium, plus licences |
Our method: assess, then strangle the monolith
- Assessment (two to four weeks, $5k to $15k): we read the code, map modules and data flows, measure test coverage and deployment steps, interview the people who use and maintain the system, and rank risks. You receive a module map, a dependency graph, a risk register and a sequenced migration plan with costs. The plan is yours whether or not we do the work.
- Test harness around legacy behaviour: before anything moves, we record what the old system does. Characterisation tests capture current outputs for real inputs, including the odd ones, so the new module can be checked against them and nobody has to argue about what the correct behaviour was.
- Strangler fig migration: a routing layer (API gateway, reverse proxy or feature flags) sits in front of the old system. One module at a time is rebuilt beside it, traffic is shifted gradually, and the old code path stays live until the new one has proven itself. Each slice ships to production on its own.
- Data migration and reconciliation: data moves with its module, with dual writes or change-data-capture during the overlap and reconciliation jobs comparing the two stores every night. Cutover for a module happens only when the counts and sums agree for a sustained period.
- Retire: when the last module is gone, the routing layer, the sync jobs and the old servers are removed, and the documentation and runbooks describe only the new system.
Using microservices for legacy software modernization
Microservices are a tool for this work, not the goal. The strangler fig pattern needs a seam to cut along, and a service boundary is a clean seam: the new booking service owns bookings, exposes an API, and the old monolith calls it instead of its own code. But every service adds deployment, monitoring and data-consistency work. We split along the lines where the business actually changes at different speeds, keep the number of services small, and leave stable parts as a well-structured modular monolith. For Paint Nite, eight NestJS services behind one GraphQL gateway replaced a platform with a decade of change, with sync services keeping the new platform and the legacy one aligned so features could migrate in slices while tickets kept selling.
Stacks we move from and to
| From | To | Notes |
|---|---|---|
| .NET Framework 4.x, ASP.NET Web Forms and MVC 5 | .NET 8 with ASP.NET Core, or NestJS where the team is moving to TypeScript | Entity Framework 6 to EF Core; Windows-only hosting to Linux containers |
| PHP 5 and 7, custom frameworks, CodeIgniter, early Laravel | Laravel 11 or FastAPI, with a Next.js front end | Session, auth and file-upload behaviour needs careful characterisation tests |
| AngularJS 1.x, jQuery front ends, server-rendered pages | Next.js and React with a typed API | Can run page by page behind the same URL structure |
| Java 6 to 8 on Struts, JSF or Spring 3 | Spring Boot 3 on Java 21, or a service-by-service move to NestJS or FastAPI | Keep the JVM where the business logic is large and sound |
| Access, FoxPro, Excel-driven processes | PostgreSQL with a web application and role-based access | Data cleaning is usually most of the work |
| On-premises SQL Server or Oracle | PostgreSQL on AWS or Azure, or managed SQL Server where licences make sense | Stored procedures move last, after the application code that calls them |
Where the target stack matters, our custom software, enterprise software and cloud and DevOps teams build it; where you want to keep .NET, you can hire .NET developers from the same bench.
What legacy modernization costs
| Engagement | Price | What you get | Timeline |
|---|---|---|---|
| Modernization assessment | $5k to $15k | Code and architecture review, module map, risk register, sequenced migration plan with costs | 2 to 4 weeks |
| Fixed-scope modernization | $40k to $150k+ | Strangler fig migration of the agreed modules, test harness, data migration and reconciliation, documentation and handover | 4 to 12 months |
| Dedicated modernization team | From $4,000 a month per senior engineer | A team embedded with yours, working through the plan in your repositories and cloud accounts | Ongoing, usually 6 to 18 months |
| Maintenance after cutover | $1.5k to $8k per month | Monitoring, patches and small changes on the new system | Ongoing |
The price depends on the number of modules, the state of the data and how much of the old behaviour is documented. Our cost guide for custom software explains how we estimate, and pricing lists every published rate.
Risks and how we control them
| Risk | Control |
|---|---|
| Undocumented behaviour that users depend on | Characterisation tests from production inputs; shadow traffic comparing old and new responses before cutover |
| Data drift between the two systems during overlap | Dual writes or change-data-capture, nightly reconciliation with counts and sums, no cutover until they agree |
| A big-bang cutover that fails | Module-by-module slices, feature flags and a rollback path for every slice |
| Knowledge leaves with the people | Interviews recorded in the assessment, architecture decision records, runbooks written as we go |
| Scope grows into a redesign | Behaviour parity first, improvements as separate, costed items after each module is stable |
| Integrations that only the old system knows how to call | An integration inventory in the assessment; adapters tested against the real third parties early |
Timeline
Take a typical mid-sized system: a .NET Framework application with 15 modules and one database. Assessment runs in weeks one to four. The harness and routing layer follow in weeks five to eight. The first, lowest-risk module is in production by week twelve. After that a module ships every two to four weeks, with data reconciliation running throughout. Most projects retire the old system between month six and month twelve. Larger estates or systems with several databases take longer, and we say so in the assessment rather than later.
Modernization work we have shipped
- Paint Nite: a national events platform with more than a decade of change, rebuilt beside the legacy system. The purchase flow had lived in one function of roughly 1,200 lines with no Stripe idempotency keys; it became a six-stage checkout pipeline with idempotent payments, and search reindexing that used to take production offline now ships with no downtime. 754 automated test files run on every pull request.
- AutoViz: a product where hard-coded catalogue schemas were replaced by a metadata system with ten value types, so new part types ship without database migrations, an example of the data-model work that most modernizations need.
- TamTracker: a market intelligence SaaS built on serverless AWS with zero-downtime deployment, the kind of target platform a modernization moves towards.
If you are inheriting a system rather than modernizing one you built, start with our guide to taking over an existing codebase, and see software maintenance and support for keeping it stable in the meantime.
Sources and further reading
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Products we've shipped, and what happened next.
Case studies written from the technical documentation of each project: the stack, the scale and the outcome.
Questions we get asked a lot.
How much does legacy software modernization cost?
An assessment costs $5k to $15k and produces a costed plan. Fixed-scope modernization projects run $40k to $150k+ depending on the number of modules and the state of the data; a dedicated team costs from $4,000 a month per senior engineer.
Should we rewrite or modernize incrementally?
Incrementally, almost always. The strangler fig pattern ships value from the first module, keeps the business running, and never depends on a single cutover day. A full rewrite is only sensible when the product itself needs to change, and even then we migrate data and users in slices.
How long does legacy modernization take?
Four to twelve months for a mid-sized application: two to four weeks of assessment, a first module in production by about week twelve, then a module every two to four weeks. Larger estates take longer, and the assessment says how long before you commit.
Do we have to use microservices?
No. Microservices are one way to cut a seam for migration. We split along lines where the business changes at different speeds and keep the number of services small; a well-structured modular monolith is often the right end state.
Can you modernize .NET Framework applications?
Yes. We move ASP.NET Web Forms and MVC applications to .NET 8 and ASP.NET Core, Entity Framework 6 to EF Core, and Windows hosting to Linux containers, or to NestJS where the team is standardising on TypeScript.
What happens to our data during migration?
It moves with its module. During the overlap we dual-write or use change-data-capture, reconcile the two stores every night, and only cut a module over once counts and sums have agreed for a sustained period.