Hello there Lizett,
Welcome to planDisney! I'm so glad that you stopped by with your question, and happy to hear you're planning a trip to the Disneyland Resort later this year.
From my experience with
standard theme park tickets, you're able to change your
theme park reservations easily through the
My Plans hub as long as it's at least 24-48 hours before your originally scheduled reservation.
Where I've seen a need to
call a Cast Member at the Tickets line for additional assistance would be in cases where someone wakes up sick on the day of their reservation and is unable to attend. In this case, the Cast Member at the Tickets line can help in finding a new date for the visit.
I would recommend first trying the My Plans hub that I linked above. If you scroll down to the second half of the page, you should see each day of your currently scheduled reservation with a quick and easy option to modify that reservation. If that doesn't work, you could try using the theme park reservations blue button near the top of the page instead.
If you're still running into issues and are unable to change those reservation dates then I would suggest
reaching out to a Cast Member at the Tickets line for more information. They'll be able to see the details of your specific situation and can let you know what your options might be.
Thanks again for your question, Lizett! I hope you have a fantastic trip to the Happiest Place on Earth. Be sure to stop by planDisney and see us again if you think of any follow-up questions. We'd love to help!
Have a Magical Day,
Greg