I know for a fact in Galveston there is rarely a cabin on any of the ships beyond the 1or 2 they leave open for emergency maintenance issues.
All those people that work in the terminal can go on the cruises on standby, if space is available. More than half the time there isnt a free (literally) cabin open for them. Do they get to go? Absolutely, from time to time. I know some though that dont even request to because the odds are so slim.
September and right after new years are the best time for them to have a good chance of going. Do further put it in perspective, if there are 5 cabins morning of, then there is a good chance. However, carnival will sell cabins right up until embark starts, so they may still be denied. 5 cabins is a lot to have open, and the likelihood of those being something you want to upgrade to are slim.
If you really want to upgrade call carnival the morning of your cruise and they will communicate with the gsm on the ship, if its a possibility to take more of your money, or upgrade your room I mean.