To achieve this recipe, we'll use phpmyadmin and the SQL language. As the following commands will modify your WordPress database, don't forget to create a backup before running any command throught phpmyadmin.
The first thing to do is getting the IDs of WordPress users. Once logged in phpmyadmin, insert the following SQL command:
SELECT ID, display_name FROM wp_users;
Right now, phpmyadmin displayed a list of WordPress users associated with their IDs. Let's say that NEW_AUTHOR_ID is the ID of the "new" author, and OLD_AUTHOR_ID is the old author ID.
UPDATE wp_posts SET post_author=NEW_AUTHOR_ID WHERE post_author=OLD_AUTHOR_ID;
That's all. Once this last command has been run, all posts from the old author now appears to have been written by the new author.