I think the worst part is the amount of man-hours it would take, not compute hours (but maybe that's what you meant?). If the change is as deep as it sounds like, the entire tooling support changed, so all scenes would have to be re-lit by artists, not-entirely-from-scratch but worse than you think.

REYES-rendered scenes that fake global illumination are pretty arcanely hacked together. Just making a legacy scene ray-traced would make it look worse, not better.

Absolutely, which is why it won't happen. You'd have to actually place lights, remove the fakes, and render check every scene. Basically the only part of the movie you wouldn't have to do is come up with dialog, sound, and geometry. (not to mention I have never met a movie person who, given a chance to reshoot a scene says "Yeah, it was perfect when I shot it, the only changes here are mechanical." :-)