Assigning posts from one user to another is always useful, and luckily for us, WordPress allows for two fairly simple methods of achieving this.
The first, easiest method for transferring posts from one user to another is by simply deleting the first user’s profile account. During the deletion process you’ll be asked whether you want to keep all items associated with that profile, and more importantly, to which existing user account you want to link these posts and comments to.
The second method involves some SQL execution, but it comes with the advantage that unlike the first method, you don’t actually have to lose an existing user account. Now to run the SQL statement you either need to fire up phpMyAdmin and hit the associated wordpress database or alternatively, search the plugins repository for something that will allow you to directly execute SQL statements from your wp-admin interface (trust me, there are plenty to choose from).
Anyway, once you are ready to go, open the user accounts page under wp-admin and hover your mouse over the two user accounts you want to make use of. Note their user-id which gets revealed in the URL of the link. Note these two ID numbers down.
Next, construct and execute your SQL statement, being sure to substitute your user ID numbers in accordingly:
UPDATE wp_posts SET post_author=NEW_AUTHOR_ID WHERE post_author=OLD_AUTHOR_ID;