Do you think if I wait to purchase theirs (I was going to buy them all before our Dec trip) I could use take their base ticket and upgrade them to an AP?
I don't think the YES tickets are upgradable.
From Chesire's ticket Sticky:
"E. I have a special ticket I would like to upgrade. Can this be done?
The only upgrades that can be done are to regular or Florida Resident MYW tickets, or Annual Passes to a higher level of Annual Pass.
Special Tickets include Touch of Magic, YES (Youth Educational Series) Program, used or unused Special Event Tickets (MNNSHP,
MVMCP, NoJ) after the date of the event, any ticket that includes the word “comp”, “complimentary”, "SPORT", or "YOUTH" on it, and any tickets that are not sold at the Park Gates cannot be upgraded or modified in any way. The last group also includes most convention tickets and the Ultimate Park Hopper tickets not sold in the US or Canada. See Post #5 for more information."