It is possible to update the online check-in with passport information. I have checked in with about-to-expire passport numbers and updated them when new passports arrived.
Even though, as noted above, passports are not required for US residents on cruises that leave and return to the same US port, entering the passport numbers should save time if your friends go on another cruise before those passports expire. When I have made reservations for a new cruise, my family’s passport numbers have automatically appeared on the confirmation.
