We got an upgrade on a cruise we booked on points, way back in 2001. From a 7A to a 4A.
That being said, the cruise line doesn't really care that you booked with points, because to them your cruise was not actually booked with points, DVC functions as your TA and pays
DCL with cash, just like you would if you booked directly or through another TA. So it really doesn't matter if you booked with points or not, you have as much or as least chance of an upgrade as any other guest.
IMHO I would think that a better option to increase your chances of a complementary upgrade would be to book a cabin in a category where there are very few rooms available, such as a cat 7 or a cat 6. But even then it is all just a craps shoot, if DCL wants the room you are in for a guest willing to pay more, or they need to do maintenance etc, you might get an upgrade, regardless of the time or category that you booked.
Based on personal experience and the experiences posted on the boards here...
Free upgrades on DCL cannot be planned for, and cannot be anticipated...
If you would not be happy in the room booked, then plan on just paying to upgrade yourself.