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.