I did this in April, and saved a whole bunch! It is a tiny bit of a hassle, like the previous poster said, you have to cancel your existing ressie's first, but you should be able to rebook within a few minutes. I saved hundreds of dollars, so it was VERY worth while!
I too cancelled my old bookings over the phone, and got little help from a rep on how to re-book online (it's a little bit of a silly system, but hey - I want my double rapid rewards credits!)
The only tricky part I found is that you have to enter everything correctly, or the system bumps you out, and you have to start the process all over again ( I hate it when you have to re-enter allyour info!) Now that I know how to do it though, I'd do it again in a heart beat! Three pointers: one, even though you aren't using a credit card to pay for the re-bookings, you have to enter the cardholder address to verify that you are the correct person using the credits. Two: you have to keep track of your Confirmation numbers in order to use the credits! So put them in a safe place. I kept hard copies, and emailed them to myself (with dollar amounts). Three: be certain to read each screen as it pops up. When the credit amounts show up - you still have to click on a little box next to each one - in order to put that credit towards the new ticket.
Good luck!