I've actually change my date TWICE from what I booked on board in November!! At first we were booked on the November 12, 2006 sailing, but realized we couldn't vacation that week, so moved it back to November 4, 2006. We since then decided we wanted to go on the West Coast Cruise, and then switched the same booking to July 9, 2005. There will be no more switching, since I've now bought airline tickets, have DL reservations, etc. It can be done, though as I've been told by my TA, the option to change the date and still receive the on-board goodies can expire at any time, or
DCL can refuse to honor the change to the on-board booking, though I doubt it would happen. If you truly are thinking of changing a date, I'd book your cruise directly with DCL and then wait before transfering it to any TA. That way, you can still deal with them directly to make the change, and once you're set, then transfer it to a TA. I wish I had done that, and I will do that when we make our on-board booking in July for July 2006.
Edited to add: please be aware that if you do change an on-board booking to another date, you are still subject to DCL's current pricing for the sailing you are interested in. If you wait a month and then decide to change, even though the sailings might be pricing out the same, the price might have increased since you booked, and you'll be held to the higer price at the time of change.