Hey there Irene,
Welcome to planDisney and thank you for your question! I'm hoping that you have a trip to the Happiest Place on Earth in your near future!
While it is possible that you might be able to change your
theme park reservation on the day of your visit - it would depend on a few factors. The main factor is that there would need to still be park reservations available for the park you're interested in. There have been a few trips where I've checked the evening before to make a last-minute change to park reservations but there were not anymore available.
My recommendation would be to
call and speak with a Cast Member at the Tickets line on the day you're trying to swap your park reservation. They will be able to see the full details of your situation and be able to let you know if you'd be eligible to change your park reservation for that day.
From my experience though, I would highly recommend trying to get your plans set as early as possible because you never know what will be available as you get closer to your trip.
Thanks again for your question, Irene! I hope you have a wonderful next visit to the Disneyland Resort.
Have a Magical Day,
Greg