If you want to remove some posts from one author to another, it is very annoy for anybody, if you know how to use PHPMyadmin or some other MYSQL managers, It will be more simple. Well, follow this.

1. Log in MYSQL by PHPMyadmin or another Mysql manager.

2. Locate the database of your wordpress, Click the table wp_users. If you have another prefix, please change it Correspondly.

3. Look up the ID value of the original author(i.e, AuthorA) using:

SELECT ID FROM `wp_users` WHERE  display_name='AuthorA';

Remember the author ID(Remembered as AuthorA_ID) from the search result, usually there should be only one record, if you get nothing or more than one records, please check your syntax or look up the author ID by user_login or user_nicename, Just like this:

SELECT ID FROM `wp_users` WHERE  user_login='AuthorA';

SELECT ID FROM `wp_users` WHERE  user_nicename='AuthorA';

4. Look up the ID value of the destination author(ie, AuthorB) using the same method as Step 3, Remembered as AuthorB_ID.

5. Batch modify the post's author to another other() using this syntax:

UPDATE `wp_posts` SET post_author = 'AuthorB_ID' WHERE post_author = 'AuthorA_ID';

6. Finished.