The films will be remastered just like they were for bluray.
No, they won't: not
really.
If you shoot on film but do all your post-production work in 2K, including outputting everything for a 2K digital intermediate, you're essentially stuck with 2K. If you shoot at 4K (or 5K, 6K, etc.) to build a 2K digital intermediate, you're essentially stuck with 2K. Again, practically everything -- from 15 years ago to today -- is finished at 2K. Of the seven movies Fox announced yesterday for UHD BD, only one (!) has a 4K digital intermediate. The rest will be upscaled on UHD BD.
The remastering process for movies with a 2K DI bears zero resemblance to what was done to bring films to Blu-ray. You're not rescanning a movie; you're re-editing it, redoing the effects, etc. (unless you just upscale it, which is what will be done in all but a tiny handful of cases). It's not unprecedented;
Star Trek: The Next Generation and
Seinfeld did something similar to make it to HD, but this will almost never happen. If
Jurassic World won't pony up for a 4K DI for theaters, why would they go to that staggeringly huge expense for UHD BD, Netflix 4K, etc. where they'll never recoup that investment?
Well, what I'm saying about remastering is true if we're talking about properly bringing them to 4K. Ultra HD Blu-ray has more in its quiver than just resolution, and HDR and a wider color gamut will make a lot of movies finished at 2K look better on UHD BD than they do on traditional BD. Fox is gone back to several of their movies and given them the HDR treatment.
As far resolution goes, though, before you respond with "no, but...", sorry, the answer is still the same. If a movie from the digital intermediate era wasn't finished at 4K, no matter how it was shot, there's a 99.9% chance it'll be upscaled on UHD BD.
Not only that but movies shot in 4k with 2k effects were still shot in 4k!
But it doesn't matter. The number of movies shot in 4K or higher with 2K effects and a 4K digital intermediate approaches zero. For this conversation, whether or not a movie was shot in 4K is irrelevant if it wasn't finished in 4K.
Using your hobbit example the special effects in the original trilogy look soft in plenty of scenes because they weren't don't with bluray in mind. Did that stop them from being released? Did people say "meh the special effects look soft!" Nope.
The effects were done in 2K, which is slightly higher res than Blu-ray.