You can re-book any cruise that currently has dates announced. Now, you could re-book December 2005 if desired.
Remember that you can always choose a date and change it later if needed. However, if you change, your new price will be based on the price of the new date at the time you select it. You will be able to keep the 10% discount and the onboard credit (at last check, anyhow). However, the base price of your vacation may be different than for the dates you selected. Prices do change at least weekly if not more often!