Yes, you can. Not very likely [for free]....but you can. Simplest upgrade is one you pay for. Now the free type is much more confusing. Much more likely if you paid for a guarantee...rather than specific...cabin. Nobody knows exactly how the free type works. Some say it depends on when you bought/how much you paid/how much business your TA does with Carnival. Others say those things don't apply...but others do.
Me....i think they use a Magic 8-ball to decide. Makes as much sense as anything else. 8)
It is possible to upgrade to a balcony suite, but you are going to have to pay the difference of what you paid for, and what you want to upgrade to. Sometime there are deals, but you have to check with your vacation planner as I did, and see what they can do for you. Remember, you are the paying customer, they want your business, and to make you return, so they WILL work for you. I upgraded from an ocean veiw to a balcony, for a mere $150, then the prices for the cruise dropped, and I got a $100 cruise credit, so basically a $50 upgrade. Hope this helps.