.Mac. said:

...but we also have to consider what's actually possible with the footage already available

I'm not speaking for Ady at all here, but he has announced in the past that he'll be going to town on ROTJ. Not as drastic an edit as the PT, obviously, but it seems to be implied that he'll be filming new elements. So we're not limited by existing footage at all.

As for at what point changes become too much, well that's one reason I specifically want the 2nd Death Star kept. The half-built aesthetic is so iconic, and I would never want that removed entirely and I'm sure 99% of fans would agree with that.
I'm also sure that such a radical change as some of us have proposed would upset a huge portion of the fanbase as well. But personally, I think it's needed to save this flawed movie. In a way, it's far more tragic a failure than the PT ever was, as it actually had some of the greatest moments in the OT in there whereas the PT was all garbage. Lucas should have shown it more respect, and it's my opinion that this is a change more in the spirit of what someone like Kershner would have wanted.