So if you were to do a transformation like that you'd cut the code and paste it into a new function. Then you'd modify that function to make the abstraction work. An LLM will rewrite the code in the new form. It's not cut/paste/edit. It's a rewrite every time with the old code as reference.

Each rewrite is a chance to add subtle bugs, so I take issue with the description of LLMs "working on existing code". They don't use text editors to manipulate code like we do (although it might be interesting if they did) and so will have different issues.