Modify content inside post before first publish

You should use save_post like the following:

function modifyOnPost( $post_id ) {
   $post = get_post($post_id);
   //do stuff here before your post is created or updated
}
add_action( 'save_post', 'modifyOnPost' );

Read more here.

Update:

To give a deeper solution you could use wp_update_post( $post, $wp_error ); inside the modifyOnPost block to manipulate your post data.