Booking a Guarantee stateroom means that you are asking for the best available stateroom in your category, and if that cannot be provided, you will receive the best available stateroom in the next available category above yours. Upgrades happen often with Guarantees, but the cruise line may not upgrade you if a stateroom in your category is available.
Also, since you may receive any stateroom in the category you booked or higher, the cruise line cannot accommodate any requests regarding location of your stateroom on the ship if you book a Guarantee. If you have specific stateroom requirements – for example, mid-ship or on the port side – then you should not book a Guarantee.
Once you have received a stateroom number, you may be able to switch the stateroom if others have opened up in the category that you have been assigned. If not, you may not switch your stateroom without paying the full difference in cruise fare from your category to the category in which you would like to stay.