I could be wrong, but if someone adds on right now with Disney, they will simply pay a prorated amount as an owner on the # of points that they added on. So, if someone adds on 50 points at BLT on August 1st, regardless of UY, they will pay an additional 5 months worth of MF's for 2010.
If they have a UY that is still in its 2009 UY, then they get those 2009 UY points as those are current UY points, but the dues would be the same for someone adding on with an Aug UY or a June UY, even though a June UY would only get 2010 UY points.