Hi there, Martha! Thanks for stopping by planDisney with your question!
A Magic Key is the perfect way to enjoy the magic of the Disneyland Resort all year long! You can upgrade a non-promotional, unexpired ticket to a Magic Key, but the only way to do so in the Disneyland app is to have partially used that ticket. So, in your case, that would mean visiting the parks with your one-day ticket and then upgrading to a Magic Key before the parks close that day.
If you want to receive the special welcome package for those who purchase a Magic Key within the first 66 days but can't make it to the parks in-person to upgrade your ticket, you could purchase a Magic Key online for the full price and save your one-day ticket to use on a day that may be blocked out for your Magic Key type. Or you could gift that ticket to a friend who might want to come along with you on your next trip.
Keep in mind that
theme park reservations are required for each day you want to visit with your Magic Key. Depending on the type of Magic Key you choose, you can hold between two and six theme park reservations at a time. Magic Key holders can make park reservations up to 90 days in advance and will use a different availability calendar than those with day tickets.
I hope this helps! Have a great time at the Disneyland Resort!
-Linda