I had booked a cruise direct with carnival for last december and we had to switch it to the february. We ended up going to a TA to cancel and rebook. She did it all right there no problem at all. We even had the same booking number. The deposit was transfered from one right to the other.
Next up Elation Aug 23 '10
then the Miracle again Feb 7,2011
That's exactly what happened with us too. They simply transferred the deposit from one cruise to another and we kept the same booking number. It was no problem. TA said it was because we hadn't paid final payment yet.