Here's our experience with booking/rebooking while onboard....
We booked a 12/2005 cruise directly w/
DCL in 10/2004. We had gotten prices from Costco as well (and as usual, Costco was cheaper). However, we were sailing in 1/2005, so we were going to try and "rebook" on board to get the 10% discount, stateroom credit, etc. I spoke to the Costco agent beforehand, and they said that they could take over the original booking anytime. For this reason, we didn't book the original res. with Costco knowing that we could always give them the reservation later. Our theory was that we would get the onboard perks applied to our reservation, and then transfer it to Costco for even further savings. In theory, this is a great plan, and for some of you it might even work. For us, however, the price for our cruise went up so much between when we booked it in October and when we sailed in January, that even with the 10% off and stateroom credit, our original quote from DCL was cheaper. Long story short(er), we transferred the original booking to Costco when we got home. Still got the original price that was quoted from Costco, but no onboard perks. I guess what I am trying to say is that if you KNOW when you want to go, book as early as possible, even if you are sailing in the forseeable future. Don't wait until you are onboard, as the price may go up in the meantime. Bring your DCL quote with you onboard, and see if they can do better. If so, take it! If not, you can always transfer your original (cheaper) booking to the TA of your choice when you return.