The odds are not good. Upgrades are rare. If the cruise is not sold out, your chances are better. Disney can sell the cheaper cabins faster, so if there are empty cat 6 or 5 cabins, they upgrade to fill those and sell the newly available lower catagories.
The best advice I have seen on these boards is to not expect an upgrade. Book the catagory you want. That way you will not be disappointed when you do not recieve an upgrade.