Start with behavior, not technology
The most valuable knowledge in a legacy system is often encoded in edge cases, operational habits, and integrations. Map those behaviors before choosing the replacement architecture.
Create a boundary you can move
An API or routing boundary lets teams replace one capability at a time. The goal is not to make the legacy system elegant; it is to create a safe path around it.
Modernize in measurable slices
Choose slices that deliver business value and reduce a known risk. Instrument each release, compare the new path with the old, and retain a clear rollback strategy.