Let's say you bought your tickets from a third-party, such as
AAA or UCT, and received a discount. (This also applies to tickets bought at The
Disney Store.) Or, maybe you have resort package tickets but you booked your package reservation before the Feb 23, 2014 ticket price increase. In either case, if you upgraded your tickets before entering a theme park, you would only receive credit for what the ticket was originally worth, not what a current Disney gate price ticket is worth. Therefore your upgrade transaction would include that overall ticket price increase, plus the upgrade cost.
However, it you used these tickets first to enter a theme park before doing your transaction, your tickets would be price-bridged to current gate price, and you would only owe the upgrade cost. Since I usually don't know a poster's ticket history without asking a lot of questions, I make the general statement that you should use your tickets at a theme park first before doing a ticket upgrade.
These is no price-bridging for tickets sold directly by Disney through their online, will-call, mail order, or Ticket Booth operations.