I've been told that Disney doesn't penalize a child for turning 10. I have heard of people taking the child and the ticket to a Walt Disneyworld ticket counter and getting it upgraded to an adult pass for free if there is still admissions left on it after the child turns 10. But I don't even know if that is necessary.
I would check with Disney and find out for sure. I had a question on some of our tickets and somebody from Disney got back to me within about a week about my question.