There's no need to register file descriptors, since these stay open during the upgrade -- the process identity doesn't change.

As far as a fundamental difference with the approach you propose: We actually tried it with an earlier system called Ekiden. We found that updating by migration was slower and more cumbersome than updating in place, but I don't believe there were fundamental issues. If you look at the related work section of the Kitsune paper you'll see a nice comparison with all known approaches, as well as Ekiden.