In order to make the changes, it must be done at the pursers desk "onboard", not when you "check in" at the "pier". Simply bring all passengers that are changing to the Pursers Desk (after boarding the ship) along with S & S cards and c.cards to make the changes. This is not an uncommon practice at all by folks in order to save $ when booking or to meet the requirements of passengers under 21 years of age at time of booking.
Have a great cruise.