Hey Jenny!
Welcome to planDisney and thank you for bringing your great question our way!
The mistake you are describing happens quite often. Please do not worry! You will be able to upgrade your
Theme Park Ticket and enjoy your visit. But, this change will need to be made in person at the
Disneyland Resort Main Entrance Ticket Booth. A Cast Member will be happy to assist you. Please be prepared to pay the price difference in full.
Also, please make a
Theme Park Reservation ahead of your visit. You’ll need that reservation in addition to your valid theme park admission to gain entry. You may make Theme Park Reservations up to 180 days in advance. And your reservation should carry over once the ticket is upgraded. Here’s how to make or modify a Theme Park Reservation, log in to your Disneyland Account and select "My Disneyland." Then, click "Ticket and Reservation Details" and select "Make or Modify Park Reservations." You’ll be able to book a Theme Park Reservation for your desired date.
Enjoy your trip! If you have any more questions, bring them to us here at planDisney!
Tiffanie