Hi, Lawrinda! You can do this in advance, but it will probably be easier to do it once you arrive at your Resort. Just make sure you add the day before you have used the last one on your tickets. The Walt Disney World
Frequently Asked Questions webpage states, "New 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."
Happy planning and have a Magical vacation!