For your ease of reference, the procedures for changing the date of visit are as follows:
※If you have a fixed-date ticket, but would like to visit the Park on another day, you can change the date on your ticket by going to the ticket booths at Tokyo Disneyland or DisneySea. The change must be made before the tickets' expiration date. You may be charged handling fees for making the changes.
※On days of high attendance when Park admission is restricted, you will not be able to change your tickets for admission on that day