Did you buy the ticket from Disney or form a third party seller?
My family member was able to change the date on her account. I was not able to do change the ticket date from my account even though I was the one who purchased it, which was strange. Yet I am able to make her park reservation. Anyway. What I do not know is how many times someone is allowed to change the ticket date? I do know a date change had to be made before the valid ticket date.
It has been awhile since I purchased one day tickets, since I am a passholder myself. I remember the days when you could just buy a one day ticket and use it any day of the year. I for sure do not remember having to select a specific date.