EyeShotFirst said:

I've done hours of work trying to get Yoda to look right on single frames. The CGI would have to be replaced to look like ESB Yoda. I don't get why they didn't just dig out the old puppet. It would've been a treat for fans.

In the Making of Jedi discussion, I seem to remember that Rinzler saw the original Yoda puppet, and it was terribly rotted. That probably wouldn't have been a treat to see, and I can understand why they recreated the puppet for Episode 1. Why they made him look different however...