Hi there, Sarah!
Thanks so much for bringing your great question to the Disney Parks Moms Panel. I can't wait for you to visit the Happiest Place on Earth this year!
I'm sorry to hear that you accidentally purchased the wrong
theme park tickets for your trip. While the current special-offer tickets do expire May 23, 2019, you can most certainly take your unused ticket and put its cost toward the purchase of your new tickets when you arrive. Just head to any of the Main Entrance ticket booths to have the value of these tickets applied toward the new ones. As long as the new tickets you are purchasing are equal or higher in price than what you paid for your previous tickets you can apply the value. This means you can definitely purchase a new 3-Day Park Hopper Ticket with
Disney MaxPass while just paying the difference in price.
Before you arrive, I recommend downloading and familiarizing yourself with the
Disneyland app. Since you will be using Disney MaxPass, you will want to link your tickets to the app so you can start making Disney FASTPASS selections as soon as you enter the parks. With this handy app, you can also see attraction wait times, Character locations, and you can mobile order food and beverages from select Quick Service restaurants in both parks. This app will be your one-stop-shop for all things Disneyland Resort during your trip, and it will truly help you maximize your stay.
I hope this helps, and I wish you a Zip-a-Dee-Doo-Dah day!
-Jennifer