WordPress customizing a walker menu class

If you want change only CSS classes, you can use filters
nav_menu_submenu_css_class (for <ul> element, codex) and nav_menu_css_class (for <li>, codex).

Example of usage:

add_filter('nav_menu_css_class' , 'special_menu_item_class' , 20 , 4);

function special_menu_item_class($classes, $item, $args, $depth) {
    // add or replace classes
    $classes[] = 'rd-nav-item';
    if ($args->has_children) {
        $classes[] = 'rd-navbar--has-dropdown';
    }
    return $classes;
}