Stop in for a cup of coffee

This mail in ballot this year should be really interesting with the way the mail is right now. PA said you can change your vote. How the heck you do that if your vote is a anonymous mail in?
Somehow they can identify your vote. What I heard was you could vote in person and your mail in would not count. So, that would let you change it. You can get a mail in and not use it and still vote at the pole. I don't see any way this is truly anonymous