You have to cancel what you have and book the other reservation. I would suggest doing this from a computer and not the app because you can chose to cancel the original while booking the new reservation all in one process and it will actually hold your original reservation until you have confirmation of your new reservation. The mobile app is suppose to do the same thing but I've heard people have lost their original and ended up with no reservation because the mobile system didn't hold on to their original like it is suppose to.