How is AI changing the process of development in legacy codebases? Is rewriting the whole thing becoming cheaper than maintaining it?

I haven't seen a lot of total rewrites, but definitely carving out pieces and rewriting them with clear specs and manageable scope and modern tech stack is happening, at the end of the day AI is not magic, you still need to give it very clear instructions specially when replacing existing components and little to no room for it to go rogue, but overall definitely making a difference in this area.