Hi there, Aisya, and welcome to planDisney!
I'm thrilled to hear you're planning a trip to the
Disneyland Resort, and I think you're making a great move in considering an upgrade to Park Hopper tickets. I'm a huge proponent of them mostly, so you have the freedom to walk across the esplanade between parks whenever you like throughout your day.
I'm happy to report that you can upgrade your
One Park Per Day tickets to Park Hopper tickets. However, it's not something you're able to do on your own but will need the help of a Cast Member or your travel agent. Luckily, since the wonderful Cast Members handle this process, I have not run into issues with theme park reservations or dining reservations as they are carried over by the Cast Member making the change.
I would recommend as a first step to contact your travel agent, or you can
contact the Disneyland Resort Tickets line to speak to a Cast Member. Otherwise, this process is typically handled at a ticket booth outside the entrance to the parks once you arrive at the Disneyland Resort.
As for your visit, the
Disney100 celebration is in full swing, which makes this year such a great one to visit the Happiest Place on Earth! My top recommendations are to make sure to catch
World of Color - ONE,
Wondrous Journeys, and the
"Magic Happens" Parade. Also, be sure to read through the
Foodie Guide on the Disney Parks Blog to make a (long) list of all the snacks you want to try.
Thank you so much for your question, Aisya! Please come back and see us here at planDisney with any other follow-up questions you might have for your trip.
Have a Magical Day,
Greg