I've never had to do it but I'm almost certain that if you wish to add someone to your cabin at a later date, you may at the then current pricing, as long as there is space at your muster station. Unlike regular hotels, there has to be limits so that there is space for everyone on a lifeboat in case of emergency. I'm not sure if they allow a simple name change without any price change.
If you're going to drop someone off the reservation, it's best to do it before the final payment due date in order to not pay any cancellation fees.