Most likely the reservation hasn't been ticketed by the cruise line. If they booked a "flexible" fare through the cruise line, it won't be ticketed until after final payment or 30 days before departure. Once it is ticketed, the ability to upgrade may appear.
Note: This has nothing to do with paying the cost of the airfare to the cruise line. You may have paid for the airfare to the cruise line, but the cruise line has contractual agreements to delay payment until as close as 30 days from departure. This has the advantage to the passenger of often being able to change and/or cancel the flight without penalty while locking in the price of the airfare.