For large changes that are not straightforward and include architectural decisions, I wouldn't trust Claude enough to not read most of the code myself. I'll have to read it to be able to understand it and ask about the decisions in detail anyway. And when I start to understand it, it's not uncommon to find out that the solution can be improved and simplified in many places, and after iterating, 25-30% of code disappears.

And trying to just hand-wave it to Claude, to somehow "improve it" or "simplify it", without detailed questions hasn't been very successful. It can work for some things, though.