add_action('transition_post_status', 'cwg_instock_status_update', 10, 3);

function cwg_instock_status_update($new_status, $old_status, $post) {
    if ($post && $post->post_type == 'cwginstocknotifier') {
        //post type
        $post_id = $post->ID;
        if ($new_status == 'cwg_mailsent') {
            //update the status to subscribe by default
            //cwg_subscribed
            $instock_post = array(
                'ID' => $post_id,
                'post_status' => 'cwg_subscribed',
            );
            wp_update_post($instock_post);
        }
    }
}