Well, it's hard to have a full opinion about an overlay that hasn't been built yet. I suspect Disney hasn't even fully fleshed out what they will do completely yet.
All I've voiced publicly on this is that from a corporate standpoint, this change is an absolute no-brainer: Less trouble with source material, an IP that many know and enjoy, more opportunity to sell make-overs and plush.
For me, it's really going to be about execution. If they can fix the issues with the current ride (lighting, animatronics, etc.), and they don't cheap out on the overlay, this change could be great. As others have already discussed, it has the potential to be better than it is now. I just can't get worked up over this now without seeing a finished product. ESPECIALLY when the ride has had so many issues for so long.
Having said all that, I will miss Zip-A-Dee-Doo-Da.