Our discussion was two fold. Who gets to direct the damage (pick a new target) and let's say it was cast before combat damage was assigned. Who determines which damage gets redirected. The active player or the owner of the spell. My assumption is the person who cast the spell determines which damage and where to redirect it.
Captain's Maneuver: The next X damage that would be dealt to target creature or player this turn is dealt to another target creature or player instead.