There is a Guest Relations inside** and just outside the gates at all of the theme parks (including Epcot's International Gateway.)
Also at the Water Parks.
Also at TTC.
Also two locations at DTD.
And (as was mentioned) you can upgrade tickets at any WDW Resort, at the Concierge Desk.
The resort locations cannot do anything in regard to AP's, including upgrades to AP's.
**
MK inside-the-park Guest Relations is at City Hall in Town Square.
Epcot inside-the-park Guest Relations is on the east side pathway around Spaceship Earth.
DHS inside-the-park Guest Relations is to the LEFT of the gates as you just enter the park.
DAk inside-the-park Guest Relations is to the LEFT of the gates as you just enter the park.