Wordpress

Disclaimer: This idea requires access to the shell on your webserver.

This is the SQL query you’ll use:

UPDATE wp_posts 
SET post_author='[NEW ID]' 
WHERE post_author='[EXISTING ID]';

I had posts that were assigned to Admin (which by default is user ID 1), and I wanted to assign them all to my user (which in this case is user ID 2).

My query looked like this:

UPDATE wp_posts 
SET post_author='2' 
WHERE post_author='1';

That will go through the posts database and update the author for all of them.