Hello, dear Mary!
Welcome to planDisney, and thanks for sending your question! The weather is perfect at the Disneyland Resort in late March - you'll have a blast! Since I can't be sure of your exact challenge right now, I'll try to help you in two ways, okay?
My first guess is that you already have a ticket, and you need to change your theme park reservation. This is super simple! You would access your Disneyland account on a browser and choose the option "Ticket and Reservation Details" under "My Disneyland" (on the top right corner of the website). On this page, you'll be able to check availability for your March dates, see your current reservations, and cancel them so that you can make new theme park reservations.
My second guess is that you may be trying to upgrade your tickets to a Magic Key Pass (I did that last August). Eligible Disneyland Resort theme park tickets can be upgraded on the Disneyland App, and it's also simple. This operation would cancel your existing theme park reservations and you'd have to make new ones (don't forget to take a look at the access calendar for each Magic Key Pass before you choose the best option).
Once you find your Magic Key Pass, simply open the Disneyland App, go to "Tickets & Passes" and select the ticket you want to upgrade. Then, choose "Upgrade to a Magic Key Pass," and follow the steps until you're a fellow Magic Key Pass member!
Please note that not all tickets can be upgraded to Magic Key Passes. If your ticket has expired, or if it's a promotional ticket, like the 3-day Southern California resident special, this operation won't be available to you.
I hope these scenarios cover your needs, Mary! If you still have questions that require immediate attention, you can get in touch with the Cast Members at Disneyland ticketing. If you have other questions about planning this fun March adventure, please let us know. We love to help!
May the magic be with you,
Julie