The models are completely different between the two films, actually. The ESB walker appears a bit taller (higher up an AT-AT's legs than the Endor model would be), sleaker, and its "chin guns" were significantly longer actually, than the ROTJ model. The biggest difference is in the length of the cockpit "head" from front to back. It's a bit longer (sort of giving it a more sleak appearance) than the ROTJ model which is a tad more squat.
Ultimately the model makers intended them to be the same I'm sure, but the models were different by the time ROTJ's filming started.
Not to say a winter deco of the new sculpt wouldn't look cool and I imagine it'll happen at some point, but the differences are significant enough to me that I consider them different models. This one's pretty well based on the ROTJ model all around, save for perhaps the ring around the hatch which isn't right.