Although it seems like upgrades are plentiful sometimes, they truly are pretty rare on
DCL. Most of their sailings are full occupancy, and if not, they will offer CM's discounted fares close to sail date. That results in very few being upgraded.
You may hear of some who book a category 12, and are upgraded since they tend to overbook and there are not many of those staterooms.
The staterooms available for
DVC points bookings are not actually "held" for DVC but are allocated, but can sell out on cash too. They do not save rooms on each deck for DVC.
When you get to the port, if there are rooms still available they will be posted and you can inquire. You would pay a cash price to upgrade there at the port. Be advised that luggage will be delayed since your bags will already be checked with the original stateroom tags on them.
I always advise to book at least the minimum category of room that you will be happy and comfortable in.
