I doubt they have kept all the raw CG composites. Re-comping a movie that was 50% digital and had a composite in every shot except for one would cost so many millions of dollars. No one would be able to tell the difference anyway, since we would be watching it in 1920x1080 at home anyway.

If you think about it though, a real Special Edition of Star Wars could be done in higher than 4K, because of the live-action plates. Most of them were shot in Vistavision, so you could scan the plates at 8K and then re-comp the VFX elements digitally. Some people may say 8K scans are a bit overkill, but when you consider the possibilities and then look at the actual SE, which is a 1080p scan that already was partly printed from 2K (like a photocopy of a photocopy, with each generation on crappier paper), it's kind of sad. But like I said before, that was fine for the prequels so Lucas either doesn't care or can't tell the difference. Judging by the horrendous colouring of the SE, either one of those is perfectly plausible.