I think officially they reserve the right to charge a change fee within 45 days for any change - it is frequently waived if the change results in an increase in price. However, it's ridiculous and no reason to pay a change fee on a ticket upgrade, just wait until you are onsite and do it there. Just make sure you use the tickets to enter a park first before making the upgrade - do NOT upgrade at your resort when you check-in, just in case there was any discount associated with the package tickets.
Enjoy your vacation!