I believe
UT will take it back for a 5% re-stock fee and then sell you a new ticket. How many days do you have on your ticket and when is your trip? The max length for 2020 is a 7-day ticket, more than that and you will need to buy a separate 1-day ticket. The max for 2021 returns to 10-days. You can wait to add a day when at WDW, but depending on when and what park you want for the extra day, availability of park reservations could be an issue. I'd try calling WDW to ask if you can add a day in advance, they worst they can say is "no"