I do not think there is any reason Disney would use a film source if they ever do a 4K upscale of II or III.
On another note, RoTS generally has a very good reputation for its picture quality on BD. II less so, but I think the original master is probably of low quality, as an early digital film. A TPM scan might look better to some of us here, since the Bluray release was handled pretty poorly. Presumably Disney would use the negatives.