You code is working for me with (and without) some modifications on your code, to place in functions.php :
add_filter( 'nav_menu_css_class', 'menu_item_classes', 10, 4 );
function menu_item_classes( $classes, $item, $args, $depth ) {
unset($classes);
$classes[] = 'current-menu-ancestor';
return $classes;
}
If it’s still not working, verify that your are working on the right theme_location id in wp_nav_menu().
Hope it helps !