@izzythepush,
I'm going to take a wild guess. Maybe the states that allow you to switch your early vote have some system where you are given an envelope where you put your filled in ballot, your name and signature is on the envelope, and then on Election Day the envelope is opened and the results tallied by machine.
I imagine this would be the same as for an absentee ballot sent from overseas. So if a person decides to change their mind, they can just go to a voting center, give proper ID, and get their envelope containing the ballot destroyed and then be given a new envelope with a blank vote to be filled in. Then the person just turns in their newly filled in ballot.
Doesn't seem quite as daft as it does at first glance. I mean, what if someone voted for Trump a week ago, and the authorities dig up some bodies in the backyard at Mar-a-Lago?