You're right that I don't understand much of compositing and recompositing :D
What I meant is that if...
In special effects work, you shoot your individual elements (a TIE fighter is one element, a millenium falcon another, laser blasts another, star fields etc etc) on normal negative film.
...and...
For the SE, they took the individual elements and put them back together on a computer using more modern techniques (no matte lines, garbage mattes etc)
..., it means that most of the original negatives do exist. But if Disney want to use them for the restoration and...
The problem with Star Wars is that they used a highly unstable filmstock to do the original photochemical compositing and these shots are pretty much gone now.
..., they'll probably have to composite the elements again, but digitally, thus it may not look exactly the same, right?
It would be better and cheaper to them to make a restoration using original prints, unless they specifically want to redo the special effects, I guess.