Hi, Kitty! According to Disney's Frequently Asked Questions webpage, "Magic Your Way tickets can be upgraded before your vacation begins,
or, if entitlements remain on your Magic Your Way Ticket, then you may
upgrade or modify the ticket anytime in person within 14 days from the
date of first use. After this 14-day window, or if no entitlements
remain, then tickets may not be modified or upgraded. Upgrades include
extending the ticket length (i.e., adding more days) or adding any
combination of Options, including:
- The Park Hopper Option
- The Water Park Fun & More Option
- The No Expiration Option
Ticket upgrades must be made in person at any of the following locations at Walt Disney World Resort:
- Ticket windows at theme parks and water parks
- Concierge desks at Resort hotels
- Guest Relations at the Downtown Disney area."
So, as long as you choose to add the day BEFORE you activate your second day's tickets (and using your last entitlement), you can add a third day. If you use both days' tickets and then decide to go back another day, you will have to purchase full-price, one-day tickets. Happy planning and have a Magical vacation!