We book two rooms and just have guest services do the switch once we board. Kids in one room us in another. They just issue additional key cards at no charge.
In our case the bookings still have the correct names on them, they are just in the wrong rooms.
I may be reading two much into your question:
You will need to make sure that the bookings show the correct names of the people boarding the ship.
You used to be allowed to change the name on a booking for $50, but this was pre Covid. I haven’t had to do that in a while