Based on my experiences and what I have read on this Board, the upgrades have a lot to do with:
1) How full the ship is on your date of sailing -
DCL has an easier time selling the higher category rooms at the 11th hour, so perhaps they are more receptive to ugrading paid passengers in order to make space in the more marketable (affordable) rooms.
2) Whether you booked an assigned stateroom (you have a room number upon reservation) or opted for a guranteed category (GTY) status. Since the GTY cruisers aren't attached to a room, they are more likely to be upgraded first.
3) Whether or not you ASK, and when you ask. For my upcoming cruise in June, I have already been upgraded from a Category 8 to a Category 6 GTY, and was told there is a "chance" it could go even higher (based on 1) and 2) above). When you ask is important too, and the best time to ask (from what I learned on these Boards) was 60 days prior to sail date (because that is when the payment is due for existing reservations, and some folks wait until the last second to cancel to avoid additional cancellation penalties).
The Passporter Guide to DCL has some additional remarks about this topic.
