Not if they staffed the park with lower expected attendance with fewer cast members.
So what are you suggesting?
I'm not suggesting anything, you can already change your park if there's availability at the park you want to visit. I've done it myself.
I booked Magic Kingdom once. On the day I was supposed to visit, I looked at the wait times before entering and saw that it had longer lines than normal, so before I got to the park, I changed my reservation to EPCOT, which had significantly shorter wait times. No issues getting into EPCOT and we had a great day.
EDIT: To do this, you just go to "Modify Reservation" and you should be able to change it to any other available park. This option may need you to use a browser, not the app. I don't see the option on the app. You can also cancel your reservation and make a new one entirely, although, the modify reservation feature exists so that way you don't have to do that.
However, I believe if I didn't initially change my reservation and walked into Magic Kingdom, didn't have a good time due to the crowds, all I would probably have to do is talk to guest services and if EPCOT was still available to reserve for other guests, they would probably be kind enough to let me go to EPCOT. That's at the discretion of the CM - always be nice if that answer is "no", but in my experience, it never hurts to ask if that's what you want.
This only would work if you could reserve the park "today" (the day of). I very frequently reserve parks on the day of with no issues. You just have to do it before you tap the turnstile. I usually reserve right before I leave home. Of course, this week coming up wouldn't work out too well for that - but - it's the exception rather than the rule.