Yes, they do hold back ressies for booking onboard. If this is a priority ask where to go once aboard, and adjust said ressies.
Keep your current bookings , then adjust aboard. In fact, book an appointment for all if possible in advance. It is easier for
DCL to adjust/tweak an existing appointment than to add one ironically.
IOW, it is easier for DCL to adjust an existing ressie than to give a new one, that may not exist.
However, asking for pirate makeovers at 6pm for "nieces and nephews", of unknown
quantity , on Pirate night , is , ... somewhat obscene, but potentially possible. Lol.
Board ASAP, and go to BBB appts and changes, depending on where that locale may be. That particular locale can vary somewhat. Check the little sample Navigator they give you at checkin. If you don't see it ask. If you do have to ask expect the runaround, unfortunately. I'll guess BBB appointment alterations are made at BBB, deck 5 Atrium starboard.
Good Luck