The strangler fig pattern is pretty much the standard answer here and it actually works when you do it right. You don’t touch the legacy system, you just start routing specific functions through a new layer piece by piece until the old system has nothing left to do. The tricky part is identifying which module to start with, and the answer is almost always whichever one is causing the most friction right now rather than trying to migrate in some logical order that looks clean on a whiteboard but doesn’t reflect how the business actually operates.
