There are a few ways to do this, but if you are like me and you want to do it in bulk, a great way to do this is by using a simple SQL query.
If using phpMyAdmin
- Select your database
- Select the SQL tab
- type: UPDATE wp_posts SET post_type = REPLACE( post_type, “page”, “post” );
- Click Go
Thanks to neilscott for posting this query on the WP support forums.