Originally Posted by
MikeFromTokyo
Once a seat has been assigned to someone it is no longer in "the bucket" for the airline to upgrade someone else into it. The passenger can, at his sole discretion, occupy the seat or give it to absolutely any other passenger he wants to give it to.
This is not factually correct. The seat remains the sole property of the carrier. Seat swaps can be done, but the crew should be notified. A seat swap between members of the same traveling party should never be a problem, but a customer giving their seat away to some random person is something the airline staff can disallow.