It's my understanding that you can change your dates after you return home and maintain the discount and onboard credit. On our Jan 31st cruise, we booked with Donna for a date we don't plan on going. We wanted to book to go againg in 2 years, but the dates haven't been released. So, we booked it for sooner and she even made a note on the reservation that we plan to move it to Jan 2006 when the dates open up.
Donna also told us that the discounts and onboard credits for 7-day cruises are only for Eastern and Western itineraries as they stand. IF new itineraries are announced, and we chose to move to something else, she said that she could not guarantee that the discount and onboard credit would apply.