Changing Monthnum to Monthname in Permalink

i am trying to change the permalink structure on my blog , i have successfully implemented , the permalink has changed but it redirects to a 404 page.

check code.

add_filter('post_link', 'rating_permalink', 10, 3);
add_filter('post_type_link', 'rating_permalink', 10, 3);

function rating_permalink($permalink, $post_id, $leavename) {
	if (strpos($permalink, '%monthname%') === FALSE) return $permalink;

        // Get post
        $post = get_post($post_id);
        if (!$post) return $permalink;

        // Get taxonomy terms
        $month= strtolower(get_the_time('M', $post->ID));	

	return str_replace('%monthname%', $month, $permalink);
}