Hey there Shannon,
Welcome to planDisney and thanks so much for your question! The good news is that yes, you should be able to upgrade your ticket to a different date - you'll just need to pay the difference in price between the two days as you noted.
The easiest way to get this done is by
calling a Cast Member at the Tickets line. They'll be able to look up your ticket, confirm the pricing difference between your current date and the new one you'd like, and process the upgrade for you right over the phone. Just keep in mind that since ticket prices at the Disneyland Resort are date-based, the amount you'll pay will depend on how the two dates compare in demand.
Once everything is sorted, you can head over to the
My Plans hub on the Disneyland Resort website to double check that your park reservation is set for the day you plan to visit.
Theme park reservations are still required along with your
theme park ticket for entry into the parks.
Thanks again for stopping by, Shannon! I hope you end up with the perfect day for your visit to the Happiest Place on Earth.
Have a Magical Day,
Greg