I have done multiple times both refactoring of untyping JS and JS fully typed with Flow. With the latter after the Flow compiler stopped complaining one can exp

I have done multiple times both refactoring of untyping JS and JS fully typed with Flow. With the latter after the Flow compiler stopped complaining one can expect things to work. With the former despite extensive tests sometimes it took weeks to fix bugs caused by the refactoring. Granted this is personal experience and I may be simply not careful enough to do untyped refactoring, but few people I talked about that shared the experience.