From what I understand, if you buy him a ticket then it's his ticket for however long he has it regardless of age. You shouldn't have to pay up in the future if he wants to use the rest of his days.
Amps is right. Disney doesn't charge the kids for growing up. You can use the ticket as-is (for a while, at least - I'm not sure of limits there) or exchange it for an adult ticket with the same number of days on it that you had left on the original ticket. They may ask how old your child is to see if it 'fits' with the ticket - if the child was 8 when you bought it 5 years ago, are they 13 or so now? - but you won't have to spend extra money.
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