Hey there, hi there, Bianca! Welcome to planDisney. We are so happy to have you stop by with your reservation question and I'd be happy to help you out.
You absolutely can change your starting park as long as there is availability for that park on the
Theme Park Reservation Calendar. I would suggest that you double-check the calendar right before you are ready to change it, as you will have to actually cancel your reservations and then rebook them. I would hate for you to cancel your theme park reservation and then not be able to rebook your ticket for the park you want.
I'd be happy to walk you through the steps to do so on the Disneyland website. First, you will hover over "My Disneyland" and click on "Ticket and Reservation Details". There you will scroll down on the page and click "View My Park Reservations". On the next page, stop and scroll down and you will see all of your upcoming reservations. There you can click on "Cancel Reservation". From there you can then book a new theme park reservation.
Since you will be arriving in May, I suggest you make sure to take note of the
Entertainment Schedule closer to your arrival date. You won't want to miss the
nighttime spectaculars that will start later this month. I am so excited for these to finally return to Disneyland Resort!
Bianca, I hope this information helps and that the park that you wish to change reservations for is available. Note, that the calendar is fluid, so if it is not currently available, it could pop up a few days or even hours later. Just keep checking!
Carrie