I remember learning that when Pixar did a pan-and-scan version of one of their movies, they wouldn't just use a floating 4:3 frame over the 16:9 original -- they'd re-render the movie , moving items around as necessary, to get a better framing. The perfectionism was palpable when we'd meet with them.
I wonder how easily a Marvel or Starwars movie could be re-rendered like this. Technically there are actors of course, but they are usually the main focus of the scene anyway...