My understanding is if you book one date, and then change to another, they will transfer your deposit to the new date. However, you may not get the same rate for the new date. You will be subject to the current price structure of that cruise. In other words, it's not so much changing your reservation as it is cancelling one and creating a new one. The only difference is that they won't bother refunding your deposit from cruise A, only to have you re-submit it for cruise B.