We upgraded ours at the end of our trip. If you use your 21 day pass to enter a waterpark then you have to upgrade to the Premium AP, we didn't so upgraded to the regular AP for the 4 parks. We were told the AP would start from the day we activated our 21 day tickets but they didn't we got 1 year from the day we upgraded. Have a look thru the last few pages of posts, there has been a couple of threads on this subject.