From one Coloradan to another, a happy hello to you, Leslie!
I am delighted to hear that you are considering upgrading your ticket to a
Magic Key! At this time, you can only upgrade a partially used ticket (scanned at the park gates) to a Magic Key in the
Disneyland app. Since you have your tickets and theme park reservations in place for your September vacation, I suggest that you upgrade to a Magic Key on your last day in the parks. If you upgrade your three-day ticket at a ticket booth before you head into the parks on your first day, your current theme park reservations will be canceled and there may not be availability for the remainder of your trip, or those dates could be blocked out depending on the type of Magic Key that you select.
So, it's a good idea to use your tickets and
theme park reservations as you had originally planned. Then on day three, enter the parks and upgrade your ticket to a Magic Key in the Disneyland app sometime before the parks close that night. Just keep in mind that your activation/anniversary date will be retroactive to the first day that you used your three-day ticket.
I hope this helps! Have a fantastic time at the Happiest Place on Earth!
-Linda