Wordpress - How can I convert Posts into Custom type posts?

It seems that the easiest way is to do it manually in the DB. If you don't have access to the DB, or if you don't want to do it manually, you can install a plugin that will do it one post at a time, or a plugin that will bulk convert a lot of posts at once

there is a great plugin Post Type Switcher that gives you simple way to change a post type in WordPress.

For those who are looking for a code solution:

$post_id             = 123; // Set this to the ID of the post you want to update.
$post_obj            = get_post( $post_id );
$post_obj->post_type = 'custom-post-type'; // Set this to the slug of your custom post type.
wp_update_post( $post_obj );