You can change the author of a WordPress post at any time you want and here's how you can do that:
Change Author of a WordPress post
Note: Typically, you can see users with all the user roles here except the "Subscriber" and "No role for this site."
For Multiple Posts at Once
Click on the "Author name" beside the "post title" to see all the posts written by that author only.
WordPress, by default, shows 20 posts per page. If you want to see more than 20 posts on a page, go to "Screen Options" and then change the value as per your wish in the "Number of items per page." Now, click "Apply" button.
Not only you can do all the above for the existing posts, but you can also do the same for the Drafts and Pages.
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How to Change Author of a Post in WordPress