DCL has been offering lots of incentives to fill some of its ships that haven't been selling as well as they would like. Some examples for these Spring Med cruises are the kids sail free, and more recently, the $999 pp for the 1st 2 people on the cat 12 gty. The rates are $699 pp with the military offer for cat 12's. There may also be airline employee, cast member rates, and
travel agent rates too thrown in the mix. There are only 13 cat 12 rooms. DCL will far exceed selling 13 cabins at these rock bottom low prices. They do it to fill the ship. So all of those excess 12 gty people will be assigned somewhere. They could get a 12, or they could get a cat 11, or even a 10 or 9 or ???. There seems to be no rhyme or reason to upgrades or staying in your cat you booked. Check out the May 15th thread - they have a 10 page discussion on room assignments and free upgrades, plus lots of other great info!

Thy're a friendly chatty bunch!
http://www.disboards.com/showthread.php?t=2206751&page=105
I think the important thing to remember is to book the category you will be happy with, and be happy for your fellow cruisers if they get sprinkled with pixie dust and get a free upgrade!
On our first cruise, we were upgraded for free from a cat 9 to a cat 6. On our 2nd cruise, we booked a 12 gty and got a 12. Now if you really want to stay in your particular cabin and don't want a free upgrade, let your TA or DCL know that. And DCL will not move your assigned cat 9 cabin to another cat 9 cabin. If your booking is a GTY, you are guarenteed to get at least that category or higher, but it could be anywhere on the ship.
If you don't get a free upgrade, or if you go from a 12 to an 11 but would like a window, you can always check into paying for an upgrade when you board. These are generally at a discounted rate. DCL has a chart, and you pay the difference of what you booked at. So if you paid for a 12, and got a 9 for free, but want to upgrade to a 6 at port, you pay the difference from the 12 to the 6.
Hope that explanation helps. Any other veteran cruisers, please add your upgrade experiences.