I recently had a somewhat similar experience with ES. Last year we booked the Magic for a 12 day Med. cruise on Sept. 24th. After doing some research on excursions, and my wife's condition, we decided she was not going to be able to endure the excursions. We also had the Freedom booked for Dec. 2010. We planned on canceling the Med cruise when we returned from the Freedom cruise. I knew we would book another cruise, and that it would cost us an extra $100 to change to the new cruise. That was fine, because that is the risk you take with ES. While on the Freedom cruise I bought a future cruise certificate for $100. This can be used for deposit, or partial payment on a future, or booked cruise. With this certificate you also get an on board credit of $100 for a 7 day or longer cruise. When we returned home and changed our cruise we sent the certificate in, and changed our cruise to the Freedom 8 day for Sep. 24th. They took the $100 on board credit and used it for the $100 re-booking fee. So it really did not cost me anything to change to a different cruise. The key is to make sure you know what you are signing up for.
Hope you get it worked out