FYI, according to many articles I have read, if upgrading a discounted ticket, you must have used it at least once and have at least one day remaining on it before doing the upgrade.
Just to pile on - this is correct. I recently called both Disney and Undercover Tourist to inquire about the same issue, and it was confirmed that: (1) the only way to upgrade 3d-party-purchased tickets is in person at a Disney Guest Services window; and (2) if we upgrade
before using the tickets, we pay the difference between the
discounted price we paid for the tickets and the current gate price of the upgraded tickets, but if we upgrade
after using the tickets for entry to at least one park, but while they still have at least one day's entitlements remaining (in other words, before we use them for park entry on our last day), then we only pay the difference between the
current gate price of the original tickets and current gate price of the upgraded tickets, thus preserving the discount.
*NOTE: For what it's worth, Undercover Tourist DID tell me that if I'd bought the tickets but hadn't linked them to MDE yet, I would have been able to return them to UT and buy upgraded tickets from UT directly. However, by the time our plans changed, a day got added to the front of our vacation, and we decided we wanted to visit a waterpark, I'd already linked the tickets to MDE.
Disney's policy on this makes little sense to me and seems penny-wise and pound-foolish (why not allow people to upgrade any valid ticket, online on the Disney website, for the difference between gate prices -- then you get their money, rather than have them change their mind or balk at losing their discount or having to wait a long time at a Guest Services window, such that they decide not to upgrade at all), but it is what it is.
Unfortunately for me, our desire was to upgrade our 6-day MYW tickets
before they'd been used (adding parkhopper plus on day 1 of a 7-day trip, going only to a waterpark on day 1, and then to the parks on days 2-7). But if we did, we'd lose our discount, (which was about $110) and would end up paying $490 to upgrade (compared with the $260 cost of just buying 1-day tickets to a waterpark, or the $380 cost if we'd been able to upgrade
without losing the discount). Although it would be nice to have parkhopper plus and maybe throw in a round of mini-golf and an evening park-hop or two, the loss of a $110 discount in doing so (roughly the cost of a TS meal) is a deal-breaker for me.