Hi Hyejeong! Welcome to planDisney.
Your trip isn't too far off. You must be excited to experience all the magic of your Disneyland Resort vacation. I understand how
plans sometimes change, and it is possible to upgrade your unused
theme park admission ticket to a higher tier. The best way to do that is once you arrive. At the Main Entrance, a helpful Cast Member at one of the Ticket Booths can assist you with the upgrade. You'll simply have to pay the difference and prices are subject to change. You'll also need to make a theme park reservation to enter the theme parks.
However, if you're concerned about waiting to make this change until you've already arrived, I'd suggest calling the dedicated
Tickets phone line to see if a Cast Member can address your concerns.
One of the options you'll have when you enter the parks is to add the Genie+ service on the Disneyland mobile app. It's a real timesaver with its Lightning Lane Entrances. Our family found it to be a real value. We were able to fit almost everything we wanted to do into a short trip!
I hope you have a magical visit.
Elizabeth