A nice solution was used during development of the Go language: a tool for code rewriting (it was called "go fix"). It allowed the language authors to quickly and easily transform large swaths of code in the standard library, and also let everybody execute the same transformations in any third-party code. Thus enabling rapid and extremely painless iterations. [I'm consciously simplifying the story a bit to make it shorter and more concise, but I'm not hurting the core message I believe.]