We were upgraded from a Cat.6 to a Cat.4 on our Dec 2001 4-night cruise. BIG difference in the size of the cabin.
Because it was post-911, I just assumed that it was Disney's way of thanking us for not canceling our ressies. The ship was at about 3/4 capacity and the final payment had been due just a couple of weeks after the terrorist attacks. I'm sure a lot of people were still too nervous to travel at that time.
We already had our cruise documents in hand when the TA called to congratulate us on our upgrade. I think it may have been only a week prior to sailing.