One thing...
You can not upgrade to an AP at any resort. It must be done at guest relations. The resorts do not have access to the ATS (automated ticketing system), which is required for upgrading. It may also be required for renewing without trading in a ticket as upgrading. ATS can basically be found in any park (including water park) ticket window or Guest Services location, or one of the Downtown Disney Guest Services locations.
Second thing...
If you upgrade a ticket before using it, you only get the value of what Disney sold the ticket for. So, if you purchased your ticket from a discount broker, who is making a slight profit, you will get less than what you paid for it during an upgrade and lose money. Even if you purchase your tickets as part of a package, ticket prices have been going up in August the last few years, so watch the date of purchase (or package booking) and when you will use it. It is better to develop the habit of just using your ticket before upgrading.