How do I override a sidebar that is registered in a parent theme when using a child theme?

Here is what worked:

add_action( 'after_setup_theme', 'parent_override' );
function parent_override() {

    unregister_sidebar('sidebar-4'); 
    /** I have looked for the ID of the sidebar by looking at        
     *  the source code in the admin.. and saw the widget's id="sidebar-4"
     */ 

    register_sidebar(array(
        'name' => 'Footer',
        'before_widget' => '<div class="span3">',
        'after_widget' => '</div>',
        'before_title' => '<h6 class="footer-widgets-item">',
        'after_title' => '</h6><hr>',
    )); 
}

It seems that the id is not a slug of the name, and if you did not specify an id upon registering the sidebar, it will have an id of “sidebar-#”…