Welcome to planDisney, Leetice! Thank you for bringing your question to us!
It may be possible to upgrade your One Park Per Day Theme Park Ticket to include the Park Hopper Option. However, upgrading a single day of a multi-day ticket to include Park Hopper is not possible. Any ticket upgrades will apply to the entire visit. Therefore, I recommend upgrading your ticket on the first day if you think you’ll want to use the Park Hopper feature for any part of your trip. This upgrade can only be made in person at the
Disneyland Resort Main Entrance Ticket Booth and is subject to availability.
If you are able to upgrade to a Park Hopper ticket, you will have the option to hop to the other theme park starting at 11:00 a.m. each day of your vacation. You will not be required to make another Theme Park Reservation in order to visit the other park. And, you may hop back and forth as often as you like (subject to attendance availability.)
I hope you found this information to be helpful. If you have more questions, please head back to planDisney!
Tiffanie