In order to upgrade a child's PH to an adult PH for free, the "child" must be present and fit the age that would reflect that they were under 10 when the pass was originally used. In other words, you cannot bring a 14yo to Guest Services with a child's PH that was first used last year and expect to upgrade that pass to an adult PH. If WDW apply this rule, can you imagine the number of dishonest people who would purchase the less expensive child's PH and then try to upgrade it to an adult pass?
And since park passes are "non-transferrable", you cannot even pay to ugrade the pass to adult status so someone else can use it. But if you have a 10+ child who would have been under 10 at the time that the PH was first used, then there is no way for WDW to know that the child was not the original owner of the pass.
