It's official---no charge to upgrade it an adult ticket. As long as the ticket was used at least once by the child, you are set. However, if he is barely no longer a child (say he or she just turned 10), I wouldn't bother with going to guest services, just use the ticket as is. If you want to upgrade anyway, bring the child with you. And make sure you know the policy---the Guest Services inside Epcot tried to charge me. I insisted there wasn't a fee (had called Disney earlier to confirm this "too good to be true" policy)--they finally got someone who knew what they were doing and we were set. HTH