The points used to upgrade are forfeited should you rebook the flights withing 24 hours of travel or fail to board.

If you cancel a confirmed Flight Upgrade more than 24 hours before scheduled departure, the Upgrade points may be refunded less a 5000 point "Refund of Unused Flight Upgrade Fee".

I have come across anecdotes of sometimes being able to change to an earlier flight in the upgraded class at the airport on the day of travel, but that's very much a YMMV situation.