McGregor wouldn't go for it and neither would Lucas. The replacement of Anakin wasn't a big deal to GL simply because Shaw uttered like 5 lines, and having the young Anakin Skywalker, who basically dies when Vader is born makes some sense.

Guiness will not be removed. The only character I see being replaced would be in a 'alternate special edition' version with a CGI Yoda. Lucas has the utmost respect for Oz and his work with the Yoda puppet, so the only way I see him doing this is as a bonus just to show fans what it would look like.

I'm 100% behind a CGI Yoda in TPM though.