There is no sure thing when talking about up-grades, you either get lucky and get one or not. As far as I can tell, there is a slightly better chance of getting one when booking a guarantee cabin catagory. Here is a link to an article that explains how some get upgrades, should answer all your questions:
ALWAYS book what you will be happy with. The chances of an upgrade, without paying for it, are slim to none. If you did by some remote chance get an upgrade..it will probably still be the cat you originally booked but on a higher deck. Have fun!
Don't expect one, but if one comes feel fortunate. We've been upgraded on about 1/2 of our cruises. Don't know why, it just happens. Sometimes we get the upgrade when we get our documents, sometimes at the pier, sometimes from the TA.
I seem to believe (with no statistical data to support it) that if you book with a travel agent who sells a lot of Carnival trips the Carnival rep in your area may give favorable treatment to your TA's customers when upgrades are available.