Yes, you can do all of those things. When were on board two weeks ago we rebooked, but just listed my husband and I for now. We will call and add the kids next month. This way we didn't have to pay the deposit for everyone all at once.
The last time we sailed we rebooked on board and then called two months later to change the dates. When we rebooked on board, the month we wanted hadn't yet been posted to the reservations system, so we just booked as far out as they had and then called to change it later.