Custom walker in child theme

Hello,

I want to override class.WpdiscuzWalker.php in my child theme, but it seems wpDiscuz only checks the parent theme. Now I’m trying to use the filter, but nothing so far. What am I doing wrong?

function myprefix_comments_walker( $value ) {
	return get_stylesheet_directory() . '/wpdiscuz/class.WpdiscuzWalker.php';
}
add_filter('wpdiscuz_walker_include', 'myprefix_comments_walker');

I tried putting the add_filter() part in my after_setup_theme function, but even then I can’t get it to override.