This Method working on All Version of WordPress. Do this using Mysql Command (query) on PHP My Admin, Mysql Front, Heidi SQL, etc. This syntax is simple and fast.
I Assume that your table prefix is wp_, and User ID (check at wp_users) is 1.
Run The Following Query:
Update wp_posts SET post_status = ‘draft’ WHERE post_author =1 and post_type =’post’
Syntax above will Convert all Your Post to a draft. you can modify that syntax as what you want, depend on what your goal.
Pretty easy and fast.