Depending on when you do it, some of the CS agents will let you go over what you've already spent and create a credit, and some will not. We did it twice on our recent trip, the first time was on the 2nd or 3rd day of the trip and they let us put a $250 credit on the room charge account. The second time it was the morning of the last night (so full 24 hours left) and they would only let us go up to what was owed at that point. Be advised you need to keep all of the gift cards in case something happens during your trip and they decide to give you a credit for any reason. The credit will go back on the gift cards, and it could be any of the cards you used at any point, not necessarily the last card you used, or the biggest card etc. So make sure you keep them all until you get home and know there won't be any credit issued.