This is a great talk. I find that this pattern comes up over and over again: first add new code in a safe way that makes the codebase a bit messier without modi

This is a great talk. I find that this pattern comes up over and over again: first add new code in a safe way that makes the codebase a bit messier without modifying anything, then move all functionality to the new code and neuter the old stuff, then delete the old stuff. Eg this style of pattern is how to do schema migrations safely.