The ability to change your sail date depends on a couple of things, first if you have booked into a restrictive rate code and second if you are past your final payment date. If neither of these are the case then you should have no problem moving to another sail date and moving your deposit over.
If you are past your final payment date moving your sail date will be at the discretion of the cruise line.
If you are booked into a restrictive rate code the cruise line may allow you to change the date but charge you a fee to do so.