Hi, Nicole! Thanks for bringing your question to the Disney Parks Moms Panel. I'm thrilled that you are planning a visit to the Disneyland Resort!
You are correct that anyone age 10 and older will require an adult-priced
ticket. You can absolutely exchange your child's ticket for an adult ticket and only pay the difference in price. Just bring the ticket you are exchanging to one of the theme park ticket booths at the Disneyland Resort and they will gladly assist you.
It's awesome that you have purchased park hopper tickets because
Disneyland Park and
Disney California Adventure Park are so close together that hopping back and forth is really easy. Plus, a three-day ticket also provides you with one
Magic Morning to Disneyland Park. I love getting that extra hour in the park because I can ride some of my favorite attractions before a lot of the crowds set in. Just keep in mind that only Fantasyland and Tomorrowland are open for Magic Morning.
I hope you have a wonderful visit to the Happiest Place on Earth!
-Linda M.