For the 2014 WBPC we booked a 11C rather short notice. They had not been available before and then suddenly became available after the pay in full date. We were not able to chose a cabin, but were in GTY status. Just after we booked, an IGT rate became available. But since we moved an onboard booking, we came out at about the same as the IGT rate considering the 200$ onboard credit.
We were then assigned a 10B stateroom, so that was an upgrade. However, the stateroom was not in a good state at all (the floor was so wobbly that the coffee table could not stand on it properly and there were other issues) and after we let guest services know, we were moved to a 10A stateroom.
They told us that the cruise was sold out, i.e. every cabin was sold. But the ship was not filled to capacity as there were a lot of cabins with only one or two people in it and the Disney ships have a higher capacity per stateroom.
We also were on the second to last 5-night cruise on the Wonder before the WBPC in 2013 and the cruise director told us that that cruise was sold out as well.
I think besides maybe getting an upgrade if you have a GTY or an IGT, OGT, VGT booking there is very little chance to get an upgrade at the port.