If you rebook onboard, you generally get 10% off plus and OBC ($200 for 7 night cruise). Since you can easily change or cancel your reservation, many folks will go ahead and rebook one or two cruises while on the ship, and then move their dates once the plans are met.
This may be a stupid question - but what if you have a reservation booked -say when rates first are released - can you do anything on board with an existing reservation for the OBC?