Hi Mom-to-3--
If you bought the child 4-DAY PARK HOPPER for your child and he/she is now older and requires an adult ticket, bring your child to Guest Relations at any of the Disney Theme Parks or Downtown Disney and they will convert the ticket to an ADULT 4-DAY PARK HOPPER with 3 days remaining at no charge.
However, if you bought the child 4-DAY PARK HOPPER for your child and are now brining someone else with you to the park, you are unable to convert that old child ticket into an adult ticket. Walt Disney World ticket media are non-transferable. They must be used by the same person for all of the days on the ticket.
Have a Magical Vacation!