How It Generally Works:
Guests booking a guarantee stateroom/cabin will select this option when booking or have their travel agent select this option when booking. Closer to the cruise departure date Carnival will assign the guest to a deck and stateroom that fits within the selected category. It could be any room within the selected category, including one with an obstructed view if the guest selects an ocean view or stateroom with a balcony.
What is the benefit of choosing to do a guarantee stateroom?
Sometimes allowing Carnival to select your stateroom is the cheapest option available. However, from my own booking experience and comparing rates & offers, this is not always the case. I have seen instances where it is cheaper to choose a rate code where you select your room rather than selecting the guarantee stateroom.
When guests select guarantee staterooms they are giving up selecting the deck and stateroom. This could potentially mean a stateroom location in an area of the ship that you don't find ideal. Some people don't want to be near elevators or stairwells due to potential noise. Others don't want to be close to the ship's atrium because of potential noise. The major downside of allowing Carnival to select your stateroom is you do not get the chance to pick your room and location. Carnival will assign you to any room within the category you paid for which could put you in any location or deck with that type of stateroom. Furthermore, selecting an ocean view or balcony guarantee stateroom could result in getting an obstructed view stateroom. Also, there are a few rooms on ships that are considered "upper/lower" meaning one bed is down and 1 bed is up high. Guests in categories with these rooms could potentially get one of these rooms instead of having regular beds.
Yes, it is possible to get a better stateroom category than the one you selected, but this is very rare. If there is a higher stateroom category you want, you should book it from the start because the probability of getting a free upgrade to a higher category is slim.
Carnival assigns the rooms when they are ready and sometimes the assignment will be quick, but at other times it may take a while. The worst case scenario is that your stateroom is assigned the day of boarding the ship. If this happens you should print your luggage tags and at the pier a porter will look up your room assignment to properly tag your bags.
Carnival typically does not allow upgrades or stateroom changes once onboard the ship. You can check at Guest Services, but it is unlikely they will be able to change your room.
If you do not like your room assignment and have it before your cruise, you can try calling Carnival to see if there is anything they can do. Depending on the representative you might be able to get something changed, but that may or may not happen.