Hiya Jacob!
Welcome to planDisney! Unlike Disneyland Resort, there are no penalties at this time for canceling a
theme park reservation last minute. Plans change, people get sick, or you don't feel like getting out of bed. Having that flexibility is just fabulous.
Remember, however, Passholders have a limited number of theme park reservations they can make at a time. Disney Incredi-Pass and Disney Sorcerer Pass Annual Passholders can hold 5 theme park reservations at a time. Disney Pirate Pass Passholders can hold 4 and Disney Pixie Dust Pass passholders can hold 3. The risk with having a reservation you don't use is that there is a chance you used up all of your reservations and can't make another one until one is used up or the date has passed.
I recommend canceling your theme park reservation as soon as you know you will not use it so that others can grab it if it is needed. On busy days these reservations can run out so as a courtesy to other Annual Passholders release the one you don't want back into the wild! Of course, none of this matters if you end up visiting on a Good-to-Go day which are days that Annual Passholders can visit the Parks without a Theme Park reservation.
I hope this helps and if you have any more questions
let me know!
-Nimmy