We have annual passes, so speak from experience with the exception of the Magic Kingdom.
I have made a mistake and arrived at Epcot when my reservation was for Hollywood. Being at the Friendship Gate, I simply went into
MDE and canceled DH and added Epcot, waited a couple of minutes, and was able to tap in.
I make an exception of the MK as I know that can get crowded and if the dates are near a Holiday I’d not bet the farm that you’d be able to get into the park. But then, usually the reservation page would inform if the park has reached capacity one would think.
Either way, while it’s a pain to make the reservations, I’ve yet to see a non holiday day be so crowded you can’t make a same day reservation if you were so inclined.
(Edited to add, I didn’t make the link to the my Disney experience site. This is the second post I’ve made that links to sites, and I have no idea why it happens or how to stop it from happening)