Hi Ashley! Thanks for dropping by planDisney.
You can definitely upgrade your
1-day, 1 park per day ticket to a
Park Hopper ticket once you arrive at Disneyland Resort and pay the difference. Simply visit the Main Entrance Ticket Booths, where a helpful Cast Member can assist you with the transaction. I can't quote you an exact price, because the difference is dependent on several variables, such as the date of your visit, and is subject to change. However, they can let you know the price difference at the Ticket Booth, so you can decide whether to go forward with the upgrade.
I think you'll appreciate the flexibility of Park Hopper. There's so much to experience in Disneyland Park and Disney California Adventure Park, especially with the
Disney100 Celebration underway, that it's hard to choose just one park to visit. We often start the day in one park, hop at lunchtime, and then hop back in the evening. You can do that!
Have a magical day.
Elizabeth