I often change my onboard booking. I think they may charge you a fee now if you change the sailing more than once though, I think I was told that the last time I booked a future cruise onboard. Just make sure they keep the same reservation number as that is what connects your onboard credit for booking on the ship, and if you have a loyalty reduction on the cruise fare, make sure they remember to take that off the new fare as well. Some agents are great at this, others not so much.