Legacy software modernization
Replace or renew ageing systems piece by piece, while the business keeps running on them.
1- CALC-DISCOUNT.2- IF CUST-TYPE = 'G'3- COMPUTE WS-DISC = ORDER-TOTAL * 0.104- ELSE5- MOVE 0 TO WS-DISC6- END-IF.7+// Characterized by tests before the rewrite8+export function discount(order: Order): number {9+ return order.customer.tier === "gold"10+ ? order.total * 0.111+ : 012+}
What we build
Systems we modernize
We move business-critical software off outdated technology part by part, while the business keeps running on it.
Unsupported technology
Move off end-of-life frameworks, languages or databases before they become a security risk.
Monolith to modular
Split a tangled codebase into well-defined modules or services that teams can change safely.
On-premises to cloud
Move applications to managed cloud services to cut infrastructure work.
Desktop to web
Replace desktop or terminal applications with browser-based ones.
Database migration
Move from unsupported databases to current versions or managed services, reconciled at every step.
Interface modernization
A new interface on top of existing business logic, so users benefit first.
How to get started
Three steps to a plan for legacy software modernization
- 1Tell us what you needUse the project form or book a call. A few sentences about the goal is enough to start.
- 2Free technical consultationWe go through your goals, users, existing systems and constraints with you.
- 3Your planA detailed plan covering the right tech stack, architecture, timeline and budget. Then you decide.
How it works
Where the legacy code hurts
Our hotspot analysis sizes each module by lines of code and colours it by how often it changes. Choose a module to see its route among the 7 Rs.
Billing
Refactor
Large and changed constantly: the hotspot. Rewrite it first, behind the facade.
Hotspot analysis: code that is both large and frequently changed is where modernization pays back first.
Replacing a system in slices
A facade sends each route to the old system or the new one. Move routes across one at a time; any can be switched back.
- /customers→ new service
- /appointments→ legacy
- /invoices→ legacy
- /reports→ legacy
25% of routes on the new system. Users see one application throughout, and any route can be switched back instantly.
Services
Legacy modernization services
Take one on its own, or combine them in one engagement with one team.
Code and architecture assessment
A system map, code analysis and a modernization route for each component.
Re-engineering and re-architecting
Modules or services replaced slice by slice behind a routing layer.
Data migration
Data moved with reconciliation reports that prove nothing was lost.
Cloud migration
Applications moved to managed cloud services, with the old infrastructure retired.
Our approach
How the work runs
Each stage hands its output to the next, like the stages of a delivery pipeline.
- Stage 1
Understand
Map what the system does, who uses it, its data and its integrations, including the undocumented parts.
- Stage 2
Decide the route
RehostMoving an application to new infrastructure, such as the cloud, with little or no code change; also called lift and shift., replatform, refactor, rebuild or replace, per component.
- Stage 3
Safety net
Characterization testsTests that record what existing code actually does, so changes can be checked against it. that capture current behaviour before anything changes.
- Stage 4
Replace in slices
New components take over one slice of traffic at a time behind a routing layer.
- Stage 5
Retire
Migrate the remaining data, switch off the old system and archive what's needed.
What we'll need from you
Having these ready keeps the work moving.
System access
Source code, databases and environments for the current system.
Knowledge holders
Users and support staff who know how it's really used.
A decision owner
Someone to agree the route and priorities for each part.
Change windows
Agreed times for data migration and switchovers.
Who's on the project
Our team, working with knowledge holders from yours.
Solution architect: Assesses the system and designs the modernization route.
Deliverables
What you receive
Everything is handed over in your name: code, accounts and documentation.
To do 2
In progress 2
Done 2
What the documentation looks like
Every project ends with documents your team can run with. Here is an excerpt of one of them.
Route for each component
From the assessment · ordered by value and risk
| Component | Route | Why |
|---|---|---|
| Order entry | Refactor | Core process, changes often |
| Reporting | Retire | Replaced by the data warehouse |
| Pricing engine | Replatform | Sound logic, unsupported runtime |
| HR leave tracker | Repurchase | Covered by the HR system |
| File transfers | Rehost | Stable, low change |
Measuring success
How success is measured
What we report on in legacy software modernization projects. Which measures apply, and their targets, are agreed with you at the start.
Proving the data moved
Each migration run is reconciled table by table. Run it, fix what doesn't match and run it again.
Press Run to compare row counts and checksums between the legacy and new databases.
Readiness check
Are you ready for
legacy software modernization?
Five questions, about a minute. You'll see what to settle first and a sensible starting point.
? Is the system's business value and risk understood? (y / n / ?)
How to start
From first call to production
Start where you are. Each step ends with a decision, so you commit to the next one only when it makes sense.
git show v0.1
Free technical consultation
Talk through the goal, the users, the systems involved and any deadlines.
- + A clear view of scope and priorities
- + A recommended next step
- + A plan covering stack, architecture, timeline and budget
Estimate the value
What it could be worth to you
Enter your own figures. The formula is shown, and the estimate can go with your enquiry.
# Legacy run cost removed
# formula: (Licences and support + infrastructure) × share saved
Build or buy
When you don't need
a custom build
Part of the free technical consultation: when an existing product covers the need, we recommend it instead of a custom build. These are the options we weigh, alongside the tools you already have.
Does the system encode processes that set you apart?
Is there a SaaS product that fits without heavy customization?
Can the system keep running while it's replaced?
Answer the questions to see whether to modernize or replace.
Technologies and standards
Chosen for your project
Each cloud has its own migration tooling. Choose a cloud to see the services for each step.
| Assess | AWS Migration Hub, AWS Application Discovery Service |
| Rehost servers | AWS Application Migration Service |
| Move databases | AWS Database Migration Service |
| Code analysis | CAST Highlight, SonarQube, CodeScene |
Our technology radar for legacy software modernization
techniques / adopt
Architecture decision records
A short record of each significant decision, its context and its consequences, kept in the repository.
Our default. We use it on client projects unless there's a reason not to.
Our recommendations. Updated September 2026.
Patterns
- Strangler fig
- Anti-corruption layer
- Branch by abstraction
- Change data capture
Targets
- .NET
- Java
- Node.js
- Containers
- Managed databases
Analysis
- SonarQube
- CAST Highlight
- Dependency analysis
Assessment
- The 7 Rs of migration
- TIME model
In your industry
Where it applies
What this work delivers in the industries we serve.
Financial ServicesCore system modernizationAPI layers around core banking or policy systems, and phased migration of legacy applications without disrupting operations.
ManufacturingQuality and traceabilityDigital inspection checklists, non-conformance tracking and lot and serial traceability from raw material to shipment.
Questions
Common questions
about legacy software modernization
Rewrite or refactor?
We usually recommend replacing the system part by part. A full rewrite must reproduce years of behaviour before it delivers anything; incremental replacement delivers value sooner, with less risk.
Can the old and new systems run side by side?
Yes. A routing layer sends each request to the old or new component, so slices move over one at a time and can be switched back if needed.
What if nobody understands the old code?
We analyse the code and data, interview users and write characterization tests that record what the system actually does today.
How is a project priced?
Well-defined scopes are delivered as fixed-price engagements; when requirements are still evolving, we provide a dedicated team instead. Either way, the free technical consultation ends with a plan covering tech stack, architecture, timeline and budget, so you know the cost before work starts.
More in Custom Software Development
How it connects
- Relies onQuality assuranceRegression and characterization tests prove behaviour is unchanged.
- Relies onAPI development & integrationAPIs and routing let old and new run side by side.
- FeedsSoftware developmentModernized components become the base for new features.
Planning legacy software modernization work?
Start with a free technical consultation: a detailed plan covering the right tech stack, architecture, timeline and budget for your project.
Start a project