Well, I have never cruised but I have a category 11 GTY. I was told by expedia that when all the lower category rooms fill up and as it gets closer to the cruise date, they give the bigger travel agencies (like expedia, etc) so many upgrade oppertunities. This is because the lower cabins tend to sell out easier than the higher priced ones.
So from what I understand, as in my case, all of the lower cabins up to I think category 8 are full now on my cruise, thus giving me a good chance to upgrade. If it comes closer to the date and the higher ones don't sell, they will move us up and try reselling the lower ones. This will all depend on the week that you go also. Certain dates are bound to fill up, especially the kids school vacation weeks and holiday weeks (i.e. Memorial Day).
You could always check the cruise line to see whats left for your week. Or you could just bite your nails until your info packet comes.
I was also told you could walk into port without a cabin and get upgraded right there. There are conflicting stories about that so I am not quite sure if that rumor is true.
Happy cruising!
