Hi Wendy!
Your Walt Disney World Resort vacation is almost here! It is understandable that you might need to adjust your plans, and it makes sense that you would want to wait to add a day to your ticket until you are certain you'll need it. Based on the situation you describe, I assume you currently hold 4 days of
Disney Park Pass reservations for your 4-day ticket. You can only hold Disney Park Pass reservations for the number of theme park admission days on your ticket. If you decide to modify one of your reservations so that you can visit EPCOT for
Extended Evening Theme Park Hours on your arrival day, then you will no longer have a Disney Park Pass reservation for that other day. Once you upgrade your ticket to add an extra day, you will be able to make a Disney Park Pass reservation for that day.
I recommend keeping your eye on your dates on the
Theme Park Reservation Availability Calendar. It might be helpful to know that usually, Magic Kingdom Park and Disney's Hollywood Studios are the first to "sell out" of Disney Park Pass reservations, so if you are planning to visit Disney's Animal Kingdom Theme Park or EPCOT on that additional day, I think you will probably be OK to wait to add the extra day later in your trip.
Wendy, if I misinterpreted what you were asking, please feel free to write back. Have a great vacation!
Anne Marie