And if you call Guest Rewards you increase your odds that the cancel/rebook works without someone else grabbing the room. The AGR agent can cancel the paid reservation and then quickly go back in to try to grab the open room for the award. Note: There is still a slight chance that someone grabs that room after the cancellation and before the agent can reset their system to start the AGR booking process.
Also be aware that you may have to take a voucher to get 100% of your money back. If you request a refund to your credit card you may pay a 10% penalty.