Hi, James!
Welcome to planDisney, and thanks for sharing your question with us!
I upgraded a
multi-day ticket when I got my first Magic Key Pass last year, and the key to making it happen is to upgrade before you use the last day of your ticket to enter a theme park.
This option is currently unavailable because
Magic Key Passes are sold out, but it may become available again if the Disneyland Resort resumes Magic Key Pass sales. The
Disney Parks Blog is usually the first to announce good news about tickets,
promotions, and Magic Key Pass sales, so it's worth keeping an eye on their new posts every now and then!
I hope you have a fantastic trip in January, James. If you have more questions until then, please don't hesitate to come back. It's always a pleasure to help!
May the magic be with you,
Julie