Hi, Yvonne! Welcome to planDisney and thanks for your question!
I am so glad that you are looking into upgrading your ticket to a
Magic Key! You can most certainly upgrade your one-day ticket on the day that you use it--in fact, that is the preferred method. You will visit the parks with your one-day ticket and theme park reservation as you normally would. After you have entered either park, but before the parks close that day, you will be able to upgrade your ticket to a Magic Key in the
Disneyland app. It's important that you enter the park first in order to keep your theme park reservation in place for that day.
Since you are a California resident, you will still be able to choose a payment plan when you upgrade, or you can pay the difference in cost in-full. Keep in mind that going forward, theme park reservations are still required with your Magic Key. You can make reservations up to 90 days in advance and will use a
theme park reservation availability calendar specifically for Magic Key holders.
I hope you have a great time at the Happiest Place on Earth!
-Linda