IIRC, Lucas stated on the ESB commentary track he wouldn't mess with the OT Yoda. He did try to get Frank Oz a supporting actor nomination after all!
If he ever did have second thoughts about it, I doubt he wants to look up from his desk late one night, and see Oz standing in the shadows with Miss Piggy on his arm. He's lucky the ghost of Sebastian Shaw hasn't come to haunt him yet! ;)