The rewrite that replaces everything at once is the project most likely to fail. It asks a business to bet its operations on a single launch day. Modernising one module at a time trades drama for steady, reversible progress.
Start with a blueprint
Before touching code, map the old system: its modules, their dependencies, and the data that flows between them. The blueprint tells you which piece is safest to replace first and what has to keep talking to what.
Replace one module at a time
- Pick a module that is valuable but loosely coupled.
- Build its replacement behind the same interface the old system expects.
- Route a slice of traffic to the new module and compare results.
- Cut over fully, retire the old module, and repeat.
8–12 wksto the first module live in production
The goal is that on any given week, you could stop and still have a system that works. Modernisation should never require a leap of faith.
Module by module is slower on paper and far faster in practice, because you are never blocked waiting for a single, terrifying release.