If you purchased your MYW ticket at a price less than the current gate selling price for that ticket (such as from a discounter/broker or Advance Purchase) and you wish to do any upgrade, Disney will not penalize you for the discount provided the ticket has been used. We will make an "intermediate" ticket at the current gate price, charging the difference between your old ticket and the intermediate one to an internal voucher account. Then we will take the intermediate ticket, which is at the current gate price, and upgrade it with the additional days and/or options you want, and you will only be charged for the differences in the current gate price plus tax. Note if you had paid full current gate price for your old ticket the intermediate step is not done.
If you purchased your ticket at less than the current gate price, and your ticket has not been used, you will have to pay the difference between what Disney actually sold the ticket for and the current gate price of the ticket you are upgrading to. It is always best to use one day from the old pass prior to taking it for an upgrade; you will then have locked in the current gate price as the value of the old pass.
