Go to the locked sticky Everything About WDW Tickets in the TPA&S Forum, especially Post #18, which is the current chart for all ticket prices. An easy way to get to the sticky is by clicking on the link in my signature. See also Post #7 about upgrading to Annual Passes.
You do not want to get anything from a discounter where the gate value is higher than the value of the AP you want to buy. The system will not allow a refund of a price difference unless there is a managerial approval. Even if you say you don't want a refund, the system will not allow the transaction.
By definition an upgrade is taking one ticket, adding cash (or something similar) and coming up with one ticket with more days and/or options.
So you want to make sure that you will have to make some payment. Note that the price you paid to the discounter does not matter, it is the gate price which is considered.