I don't think they had to rewrite the entire OS when they switched to intel. It only took 1 person over a span of 6 months to do the "conversion", and that 1 person was just doing it for fun. It wasn't until he finished that Apple decided to use his secret project as part of something bigger. I believe the wife briefly talked about this story on Quora. You can probably find the story if you google "marklar"