Re-import loses custom flexible attributes / fields

I’ve noticed that fields that I manually set on my original import (--set=foo=bar) are lost if I reimport using import -L, unless I also pass them with —set during the reimport. Is there a way to persist custom set fields on reimport, or at the least easily back up those fields before hand and re-set them afterward?

Alternatively, is there a way to apply autotag logic to existing library content without re-importing at all? Just modify the existing fields without changing the item/album ids?