Start by finding the code you want to change. "Find all files that import from the old utils/ directory." Claude runs Glob and Grep, shows you the list, and you decide the scope together.
Then get recommendations before touching anything. "How should I refactor this to use the new API?" Once you agree on the approach, apply changes incrementally: one file or one function at a time, which means you're only ever breaking one small thing at a time, so you run tests after each step. If something breaks after touching file, you know exactly where to look. Never batch files into one change and then debug the fallout.