Hi Emily, welcome to planDisney!
I have GREAT NEWS! You are absolutely able to cancel your current
theme park reservations and rebook them with your desired starting theme parks. I'm glad you already checked the
Theme Park Reservation Availability Calendar because that was going to be my suggestion.
To cancel your current theme park reservation, you'll need to log in to your
Disneyland Resort account, select the "My Disneyland" link, and then click the "Ticket and Reservation Details" link. Select the reservation you wish to cancel and click "Cancel Reservation". You’ll then be able to select the party members who need to cancel. Once canceled, you're free to go ahead and book your new reservation!
And before I let you go, I wanted to check to see if you've made your
dining reservations? Dining reservations can be booked up to 60 days in advance, so you're definitely within your booking window. The popular locations tend to book quickly, so I recommend you hurry if you haven't booked any yet!
See ya real soon!
Jin