1. Types, tests, all the build-time analysis you can rely on 2. Gradual migrations. You run two versions for a while and make it easy for certain codebases and/

1. Types, tests, all the build-time analysis you can rely on 2. Gradual migrations. You run two versions for a while and make it easy for certain codebases and/or users to be switched between the new and the old as necessary. 3. Sometimes you just don't touch that stuff, slowly build new things with new technology and hope eventually the old thing will fall in use enough that you'll be able to delete it.