Passengers booked on a Carnival Cruise through a travel agent are directed back to that same agent by the cruise company for any changes to the itinerary. If the change is on Carnival's end, the travel agent is notified, who then notifies the passenger.
Carnival does in-house bookings and takes care of any changes directly with the passenger in these cases. If a client is not yet booked and looking for an agent, they should consider using one that is a member of Cruise Lines International Association. This agency promotes cruise travel and provides training to make sure agents are up to date on all aspects of cruise bookings. Most cruise lines, including Carnival, are also members.