I had the same question and called Uni Customer Service to find out... before this post. Metro is right, they will allow upgrades at the park. In fact, I was told that I could use all the days on my current tickets (which are same as yours, 14 day passes from Mapleleaf) and on the last day do the upgrade and get full credit for the first tickets (not prorated for the days used). What a great customer friendly policy that is!
Anyway, my question now is how do I know how much they will allow toward the annual pass- since I bought the 7 day Stay and Play with WW on it, and actually received the 14 day passes? Guest services could not tell me this, only stated I would have to have the passes scanned and then I would be told about the credit applied to the AP. If it's reasonable at all, I would love to do the upgrade and squueze in another US trip before it expires (hey, we've forgone vacations for over 2 years now- we are overdue)!
Sorry to highjack here, but do you know how much they will allow $ towards an AP or how to figure out what the difference will be?
Thanks!!!