A legacy system is not simply old software. It is software that has become hard to change, hard to support or hard to connect to the rest of the business, while the business still depends on it. Modernising it is less about chasing new technology and more about reducing that dependency risk in a controlled way.
Assess the system before choosing a path
Start by understanding what the system does today and why it is difficult to live with. The answers shape the approach far more than the age of the technology.
- Business value: which processes depend on it, and what happens if it is unavailable?
- Pain points: slow changes, security concerns, unsupported platforms, poor performance or missing integrations?
- Knowledge: is the code documented, and are the people who understand it still available?
- Data: how much data is there, how clean is it and which other systems use it?
Four common approaches
Rehost moves the application to new infrastructure, such as the cloud, with little or no change to the code. It can reduce infrastructure risk quickly, but the application itself stays as hard to change as before.
Refactor improves the internal structure of the existing code without changing what it does. It suits systems whose core logic is sound but whose code has become difficult to maintain or test.
Rebuild rewrites the application on a modern architecture while keeping its purpose. It offers the most freedom but also needs the most care, because years of business rules are often hidden in the old code.
Replace retires the system in favour of an existing product. This works when the system supports a common process that a well-supported product now handles well.
Modernise in stages where you can
Replacing a critical system in one cut-over carries a lot of risk. A staged approach, sometimes called the strangler fig pattern, puts new components alongside the old system and moves functionality across piece by piece. Users move gradually, each step can be tested on its own, and the old system is switched off only when nothing depends on it any more.
Protect the data and the business rules
Data migration usually needs more planning than expected. Map where data lives, clean it before moving it, and run the old and new systems side by side long enough to compare results. Capture business rules from the code, from documentation and from the people who use the system every day, then turn them into test cases so the new system can be checked against them.
What this means for your business
Begin with a clear picture of value and risk, choose an approach per component, and plan for gradual change rather than a single switchover. If you are weighing these options for an older system, our legacy system modernisation page explains how we approach assessment and phased delivery, and our development process shows how each stage is reviewed before the next begins.
