The original CG models used a bit in TPM and a lot in AOTC and ROTS may not have been originally rendered in a resolution higher than the original film. If they were that resuktion, then as the poster above says - They'd need to draw new ones.
If they had higher resolution models available, they could place those into shots.. if they kept the code and data to generate those shots (a lot were used in 100% CG settings) otherwise they would need to to regenerate it all. And if they kept the original raw footage.
BUT there is an even bigger problem. AOTC was filmed on new digital cameras that did not have super high resolution. Like DS9 was filmed on video and so cannot be upscaled like TNG, there is a risk that AOTC does not have the source material to scale much higher.
If I was going to revisit AOTC and ROTS for some special editions - I would probably do more significant hacking to improve the story telling. The effects are not the problem... Except Obi-Wans beard and hair continuity in the films.
UKS