The big answer is "it depends." If the sale code says "new bookings only," AND you are more than 90 days out, you can "reshop" your cruise. This basically amounts to canceling and rebooking while keeping the same cabin. If it does not say "new bookings only," you can just have it applied to your existing reservation.
If you are less than 90 days out and it says "new bookings only," you are basically dead--you'd hit a 50% penalty if you canceled and rebooked. If it does not say "new bookings," you can have the new rate or discount code applied to your existing reservation.