All I ever used to read before Animorphs was Goosebumps, and the only reason I read them was because I couldn't anything else easy or fun to read. That said, I was never engaged in any of the Goosebump books that I ever read. Animorphs was the first series that truly made me care for the story and its characters, so I would NOT want R.L. Stine to ever touch Animorphs and ruin that for me.
The only exception I'd make to this is if he wanted to do the Alternamorphs, since no one really cared for them anyway.