based on the digital bits, lucas reportedly gave serious consideration to replacing the puppet, but was talked out of it.

puppet yoda in OT occupies space and looks tangible. for a good chunk of the PT, he does not. in bails ship he looked pretty good. and on kashyyk. The fight in Episode II was probably the only true pop-culture moment in the PT, but that was just a gag to bring the house down.

replacing yoda in OT would be meaningless. he was filmed specifically based on what the puppet allowed at the time. so what if he had to piggyback luke during the training? it made for an interesting scene. in the 2004 commentary, lucas said nowadays he would have yoda bounce along side luke. i'll let you decide which is better.

I am fully expecting lucas to one day replace it. at that point, the tide will really turn against him. In the commentary he candidly admits that yoda carried ESB. He says those scenes on dagobah were deliberate and not flashy but we were so fascinated with yoda AND how he was brought to life. Maybe he was hoping for the same thing with jar jar, but it didn't work.