Oh yes it's possible. Quick story:
Last year I booked a 3 nt cruise on the Dream for dh's March birthday, just the 2 of us. In Feb, our district released next year's school schedule which showed the kids have a 4 day weekend in Sept. I quick booked a 3 nt incl kids & with friends. Since we'd be on the ship in just 1 month, 3 days, I figured I would rebook it onboard. Probably 2 weeks before the March cruise, the Sept cruise was already much higher than a 10% increase. So I knew there was no point in trying to rebook it onboard.
I believe cancellation penalties begin 74 days prior to sailing. Usually 75 days out is final payment due date so any cancellations done closer and you'll lose $. The amount varies with how close you are to sail date.
It think final payment due dates are slightly earlier for special sailings such as Alaskan or Med cruises, not sure.
If you are sailing in Apr 2012 and booking onboard for 2013, just have your TA cancel the original res once you get off the ship. It would really just be a matter of days that you hold 2 reservations for same sailing. Why hold onto it and take a chance of losing that deposit? Plus they can apply those funds to your onboard booking if you'd rather not have it refunded back to your cc.
That part is your choice.