I've redeemed AM for park tickets many times and have upgraded them each time. I had not a single problem upgrading them until this year. In the past I had just added days or options to my tickets but this year I had a child ticket that had to be converted to an adult ticket. The ticket that I received from AM was dated 2007(I ordered/received in June 2012) and when I went to do the upgrade the CM told me that I would need to pay the difference between what the ticket cost in 2007 and what the current price was....over $200

Based on all my previous experiences I had excepted to pay only the $19 difference from child to adult. Anyway, the CM had to get a manager to see if there was anyway that they could just charge me the $19....in the end that's what happened and I was very relieved, an extra $200 was not in the vacay budget. The manager told me that ticket policies had changed recently (which I already knew) and that was part of the problem. He then laughed and said he applauded my honestly about the ticket as most people would have lied and said their kid aged out of the ticket and got it upgraded for free. That kind of irritated me a little because if they were changing the ticket for free for most people then why did I have to give them another $19...wasn't the $4500 I spent down there enough?
So anyway....yes, you can upgrade your tickets but just be fully aware of what the prices and rules are with regard to doing it. I read many, many times that you should use your tickets to enter a park before doing any upgrades to them. I would highly recommend doing this....it certainly can't hurt and it may be the easiest way to do the upgrade without any worries. I had done this in the past but couldn't this last time.