Accessing parameters when adding filter

When you call add_filter(), set the fourth parameter to 3 (which is the number of parameters accepted by the callback function which in your case is change_rating_output()), and then change your change_rating_output() function so that it accepts the $rating and $count parameters:

add_filter('woocommerce_product_get_rating_html', 'change_rating_output', 10, 3);
function change_rating_output($html, $rating, $count){
    // Now do what you want with $rating and $count
    return $html;
}

See http://developer.wordpress.org/reference/functions/add_filter/ for further details.