You can change a park reservation for any of the days, as long as there is entry availability at that park. Typically, only the busiest times of the year will have a particular park locked out for making a reservation. But if you enter a park on day 1 of your ticket, you should be able to adjust the park reservations the rest of your trip for the other 3 parks, if necessary. Changing a park reservation is easy to do through the app.