The official policy is that you purchase the pass that is age-appropriate for the child at the time of travel. In your case, this means an adult's AP for your child. If you had purchased a child's AP for your child and had activated it within the last year, you would not need to upgrade to an adult pass if the time period on the pass had not yet expired. The same is true for unused days on a child's Park Hopper pass. Disney does not require that unused days be tossed away.