From Cheshire Figment's ticket sticky at the top of the page:
"Disney does not penalize children for growing older. If you have a partially used child's ticket, and the child is now 10 or older, you may exchange the child's ticket for an adult ticket with the same remaining entitlements and no charge.
The child for which this is being done must be present at the ticket window when this is being done. The age difference must be reasonable! For example, if the child appears to still be a pre-teen, and the ticket is two or more years old they should be no problem. However, if the child appears to be 16 and the ticket is only a year old this will not be done.
This applies whether the ticket is MYW or pre-MYW.
However, if the ticket has never been used, the ticket may only be upgraded to a ticket with at least the same entitlements and the guest must pay the difference."
So there shouldn't be a charge to upgrade to an adult ticket as long as the ticket is partially used. At least that's my understanding.