If there are still cabins available, upgrades may be available at the port but they won't be free. After completing your check-in, you can go to the upgrade desk and check whether any upgrades are available. (If the ship is full, there will be a sign saying something like "Captain Mickey reports that the ship is full and no upgrades are available.") The cast member at the desk will look up the level of the room you booked and consult a document listing the upgrade costs for other cabin levels from that level. The upgrade cost is usually less than the cost would be if you changed your reservation in advance.