Default one users posts as a different author?

I’ve got a freelancer posting some of my content to the website. So the posts default to being authored by him.

Currently, I’m having him change authorship manually. But I’d prefer to avoid the chance for error. Is there a way to:

1. Have every post he makes default to me as author, OR
2. Mass change every post he’s written to me

I know you can delete a user and assign all their posts to another. Is there a way to do this without deleting a user?

Thanks!