If you get down to the parks and decide you are having too much fun and don’t want to stop, you can add days to your Disney World theme park ticket. Each ticket can be good for up to 10 days. If you decide to add days, you need to visit the ticket windows or Guest Relations BEFORE you use the final day on your ticket. You must have at least one unused day to add additional days to your park ticket.
The great news is that you pay the difference between the number of days you have and the number of days you want to upgrade to. So if you had a 6 day park ticket, you would pay approximately $15 to upgrade to a 7 day park ticket.
Just make sure that you upgrade before the last day of your ticket is used. They make no exceptions to that rule!