Well buzzz1, that's something I was wondering also. I have called Disney & have spoken to about 7 different people in different locations. They all say that we have to actually go to Guest Services at a park to find out exactly how much it will be. They do say that it will be "prorated". I explained that the cost of the upgrade will determine whether or not I make the trip, but they can't help over the phone! Seems like somebody should be able to help us with an exact $$$ figure. If you do manage to get some info, will you please post it here? My Seasonal Pass renews in January also.Thanks.
Good Luck!