I think I read somewhere, probably Digital Bits, that Lucas planned on replacing Yoda with the 2002 CGI model from the prequels. They did tests and everything but apparently it looked so horrendous that even George had to give up, or, more likely, he ran out of time and would've done it the next release if he hadn't sold it. That would've been a nightmare change for me. Also, if they added some terrible scene where CGI Yoda "saves" Luke from the Vader in the cave and has a CGI lightsaber battle with him would've made me very mad as well.