Yes, but Disney just changed rules a little on changing tickets. I don't kow if this applies to AP or not - but you can't add no expire or extra days to MYW tickets that are all used up. This rule may not apply to AP's, but just in case, I would change the ticket before your last day.
Even if technically you can do it once your ticket is used, I've FREQUENTLY encountered CM's who are only half aware of all the rules, SO id' do it mid trip to avoid any hassle or potential problems. It's alsoways beenthe case that you had to modify tickets within a 14day window of first use.
Further - in my expereince, the BEST places to modify tickets are the guest services inside MK and the one inside Epcot. The ones at the FRONT gates of thoe two parks are also pretty good. Not sure about HS. Avoid AK. Avoid the back exit of Epcot! I'd also avoid trying to modify your tickets at the resorts, unless you are pressed for time. Last time I had to do it there and it took FOREVER!
Also, lines at guest services vary in length dramatically over the day. Lines are long in the morning, but are very short later in the day.
