When you arrive at WDW, go to a Guest Relations and tell them that you changed your mind and
that you want to buy full-price tickets instead of using your YES tickets.
It can't hurt to ask nicely, and it may very well work for you.
It's not really an "upgrade," but simply trading in new, unused tickets
for higher-priced tickets.
In fact, you can try to call Disney right now
(407) 566-4985 (option 5)
Hours: 8:30am-4:30pm ET Mon-Sat
and ask to do what I just suggested...
before your trip.