Change class-name on “current_ancestor” in wp_nav_menu

I believe the current-ancestor etc. classes are not added by the walker itself, but rather by wp_nav_menu which is calling _wp_menu_item_classes_by_context. Thus, your best bet is probably to add a filter to nav_menu_css_class and remove/replace current-* classes as needed. In its most basic form, something like:

function my_navigation_class($classes, $item){
    foreach ($classes as $idx => $class) {
        if ($class == 'current_ancestor') {
            $classes[$idx] = 'sel';
        }
    }
    return $classes;
}
add_filter('nav_menu_css_class' , 'my_navigation_class' , 10 , 2);