Tracy, was your cruised booked directly through Carnival or through an agency? If it was through an agency Carnival will not speak to you about the booking, you will have to talk to the agent that booked it.
Now to your question: During the embarkation process at the port you can ask the person who checks you in if there are cabins available for an upgrade, if they cant answer you, go immediately to the Information desk once you board and you can ask there. If there are cabins available then yes you will probably be able to upgrade, but do keep in mind that it could be quite pricey depending on what is available.
If this was booked through an agent try calling them before you leave and get them to check on an upgrade for you, you might be able to take care of this before you even leave. If booked directly through Carnival, then call them and ask if there is availability to upgrade. They will take your payment by credit or debit card and give you the new cabin number, this way you can change the cabin number on your luggage tags before you even leave home. They will advise you to re-do your funpass and you will just take it with you to the port.
Hope this helps and good luck!!