Would they render it all out again?
There are very few, if any, big films that currently render out 4k FX and I'd say very few will ever, or for the foreseeable, re-render their FX as it's too expensive.
The problem with both
Attack of the Clones and
Revenge of the Sith is that, unlike The Phantom Menace being shot on 35mm film, the films themselves were both shot
1080p and matted to 2.39: 1, so they can never be truly 4k, or even reach
true 2k – so it's upscale all the way.