I don't think anyone really has a great idea but from what I can tell it isn't that often that it happens. We did get one on our 5-1 Eastern as did some other people on that sailing but from what I've gathered here in the last few years they are few and far between. Hubby and I have been on over 15 cruises with and without kids and on other lines besides Disney and this was our first ever complimentary upgrade. I certainly wouldn't count on it and would make sure you book a room you are happy with. Most people on here seem to agree that the people who book the lower categories ie 11, 10 are more likely to get upgraded so that Disney can sell the lower categories and sell out a ship but I don't think anyone ever knows for sure. We had booked 2 connecting cat 8's and got upgraded to 2 connecting cat 4's so I don't know if the lower category theory always applies either.
Heidi