Yeah that's why step 0 is to tell the LLM to write a test suite for the thing you want to rewrite. It'll do that, but it does require adult supervision. Once you have that test suite, you can send the LLM into an inner dev loop. Like human programmers, LLMs are great when they have a fast inner loop.

The fact that LLMs exist doesn't mean you can just stop thinking. It mean the things you have to think about will be different. You have to treat them as savants with absolutely no ambition.