At that point you create an entirely new API, fully versioned, and backwardly compatible (if you want it to be). The point the article is making is that AI, in theory, entirely removes the person from the coding process so there's no longer any need to maintain software. You can just make the part you're changing from scratch every time because the cost of writing bug-free code (effectively) goes to zero.

The theory is entirely correct. If a machine can write provably perfect code there is absolutely no reason to have people write code. The problem is that the 'If' is so big it can be seen from space.