We can always hope that the TNG crew will be able to scan the VFX elements (those that were filmed on 35mm, that is) in at HD and recomposite them. They'll need to generate new things like phaser beams and any CG work that was rendered in 480i, but in principle it's possible to keep most of the original ship elements.
My preferred solution, in fact, would be for them to use as much of the Enterprise model work as possible and mix it with new CG ships to replace those Excelsiors and Mirandas we kept seeing over and over and over. That way, CBS-D could focus their efforts on fewer CG ships and have a benchmark with the model work to match. I suspect we won't be so lucky, however.
Here's a question. How many of the ship shots were actually done on film? I know that all the ILM stuff was, and I know that it was all composited on tape. However, many of the ship shots, mainly the guest ships of the week look like they were also shot on video, as well as a lot of the later season shots of the four footer. Honestly, if they were going to go back and re-composite the original f/x elements with "fill-in" CGI work here and there, THAT would get me really excited about the project, but I don't know how much of it was actually done on film to begin with.

. TOS, and Season 1 in particular, are full of these types of shots. It's not just the shot composition, but the use of colored lighting as well. Look at the conference room lighting, with the use of blue gels, and the shadows behind Mudd and his women. It adds depth and detail where in reality there was none. It looks fantastic, and I'm pretty sure those aren't even 1080p screengrabs. TOS really was a revelation when it was remastered, and that was the real benefit of the TOS-R upgrade. Seeing the original live-action footage with such brilliant color was incredible. They weren't shy in those days with using "hyper-real" lighting, unlike a lot of the drap "realistic" lighting used in more modern shows. That's why I suspect that when TNG makes the jump to HD it won't be as eye-opening as TOS was. The color palette and lighting just isn't as bold. It will look good, just not in a "wow I had no idea it could look that good" way.