add_filter('post_link','wp_automatic_permalink_changer');

function wp_automatic_permalink_changer($permalink ){

	global $post;

	if (!empty($post->ID)) {

		$link_to_source = get_post_meta($post->ID, '_link_to_source', true);

		if ( trim($link_to_source) != '' ) {

			$new_permalink = get_post_meta($post->ID, 'original_link', true);
			if(trim($new_permalink) != ''  ) return $new_permalink;

		}
	}

	return $permalink;
}