Hi there, Jose! Welcome to planDisney. Thanks for bringing us this great question!
Park Hopper tickets are my favorite kind, especially at
Disneyland Resort where the parks are so close to one another. On my most recent visit, my kids and I timed ourselves and it took us exactly 94 seconds to walk from gate to gate!
Jose, unfortunately the only way to “switch” your starting park is to cancel your reservation and create a new one. Given your trip is just a few days away, you’ll want to be absolutely sure that the other park is available on August 18. You can check the
Theme Park Reservation Availability Calendar. As of this moment I see both parks available, but availability can change in just a few seconds, so you do run the risk that between the time you cancel your current reservation and make the new reservation, the date may not be available.
If you decide you'd like to cancel your reservation and make a new one, go to the
Disneyland website and log in with the account you used to purchase your tickets, go to My Disneyland, Ticket and Reservation Details, View My Park Reservations. Next to each reservation that you have, you'll see a Cancel Reservation link. Once you have cancelled, you will be able to go to My Disneyland, Ticket and Reservation Details, Make Park Reservations and make a new reservation.
Keep in mind that with a Park Hopper ticket, you can move back and forth between both parks as much as you’d like after 1:00 p.m..
Sending you a pinch of pixie dust for good luck! I hope your visit is magical and memorable, to infinity and beyond!
Kamini