There are two types of upgrades.. the one's you pay for, and the magical ones you are surprised with
Sometimes as a sailing date approaches, and if the ship is not fully booked, you can pay for an upgrade and can get a good deal on it. Let's say you book a CAT 8 Deluxe Ocean view, and call and they tell you, "Would you like to upgrade to a CAT 6, Deluxe Stateroom with Verandah for an extra $50 per person.. and you jump up and down and say HECK YEAH I WANT THAT UPGRADE). Sometimes you can get a good deal on an upgrade with you check in, it all depends on how booked the ship is, and which CAT's are still available at sailing.
Other times, they might overbook a Category and will need to upgrade people, so those are the free surprises.. or if a ship isn't that full, they will "upgrade" people to a different deck and higher category, just to balance out the numbers in the staterooms. That way they can close down entire sections of staterooms, rather then have odd ones empty here or there. I would assume that on
DCL, it is more likely to get the free upgrade because of overbooking.. since I have been told that most DCL cruises sail at or near capacity.
Hope that helps. Best advice is to book the stateroom category you will be happy with, rather then counting on a a cheap or free upgrade. That way you aren't disapointed.