Hey there Chanel,
Welcome to planDisney and thanks so much for your question. What a fantastic way to celebrate your son's birthday!
You absolutely can reschedule your visit to the Happiest Place on Earth. Currently, to visit one of the parks, you need both valid
theme park tickets and a
theme park reservation for each day you plan to visit.
Depending on when you purchased them, your theme park tickets should have a pretty wide window of time to be used before they expire. My favorite way to double-check the expiration date for your theme park tickets is by looking up the confirmation email you received when you purchased them. Just search "Disneyland Confirmation" in your email inbox and you'll see it a few paragraphs down in that email.
If you end up needing to reschedule your trip, all you need to worry about is your theme park reservations! Those can be changed easily from the
My Plans hub on the Disneyland Resort website. After you log in, hover over the "Theme Park Reservations" bubble and follow the prompts to modify those existing reservations.
When you're ready to make new reservations, you can make those again on the My Plans hub using your existing theme park tickets. Hakuna Matata!
As for when to cancel your theme park reservations, it's always easier if you're able to do that before the day(s) of your trip, but - as a fellow parent of little ones, I understand that you can't always plan ahead.
If you get to the day of your visit and aren't able to make it, I'd recommend just
calling a Cast Member at the Guest Information line when you get a chance. They should be able to help get your non-used reservations sorted out and walk you through how you'll need to book your new reservations when you're ready.
I hope that helps, Chanel! Thanks again for your question, and have a fantastic trip!
Have a Magical Day,
Greg