Sidebar Widget Registration without a name, how is it assigned to new named sidebar widget?

In my current theme version, I’m registering sidebars with names. However, in previous versions, I only had one sidebar and registered it without a name like so…

if ( function_exists('register_sidebar') )
    register_sidebar(array(
        'before_widget' => '<div class="menu side %2$s">',
        'after_widget' => '</div>',
        'before_title' => '<h4>',
        'after_title' => '</h4>',
    ));

But now in my updated theme, I register several sidebars and give them all a name. However, when I update the old theme to the latest version, the sidebar widget that was registered without a name is seemingly getting arbitrarily assigned to one of the new “named” sidebars, but not the one I need it to. I need that unmamed sidebar widget to be reassigned to the one named “Sidebar” in the updated theme.

Here are my new sidebar registrations…

if ( function_exists('register_sidebar') )
    register_sidebar(array(
        'name' => 'Featured_Home',
        'before_widget' => '<div class="featured-home">',
        'after_widget' => '</div>',
        'before_title' => '<h2>',
        'after_title' => '</h2>',
    ));

if ( function_exists('register_sidebar') )
    register_sidebar(array(
        'name' => 'Featured_Inside',
        'before_widget' => '<div class="featured-inside">',
        'after_widget' => '</div>', 
        'before_title' => '<h2>',
        'after_title' => '</h2>',
    ));

if ( function_exists('register_sidebar') )
    register_sidebar(array(
        'name' => 'Featured_Category',
        'before_widget' => '<div class="featured-category">',
        'after_widget' => '</div>',
        'before_title' => '<h4>',
        'after_title' => '</h4>',
    ));

if ( function_exists('register_sidebar') )
    register_sidebar(array(
        'name' => 'Sidebar',
        'before_widget' => '<div class="menu side %2$s">',
        'after_widget' => '</div>',
        'before_title' => '<h4>',
        'after_title' => '</h4>',
    ));

if ( function_exists('register_sidebar') )
    register_sidebar(array(
        'name' => 'Home_PreFooter',
        'before_widget' => '<div class="footer-pre-home">',
        'after_widget' => '</div>',
        'before_title' => '<h4>',
        'after_title' => '</h4>',
    ));

if ( function_exists('register_sidebar') )
    register_sidebar(array(
        'name' => 'Inside_PreFooter',
        'before_widget' => '<div class="footer-pre-inside">',
        'after_widget' => '</div>',
        'before_title' => '<h4>',
        'after_title' => '</h4>',
    )); 

Try moving registration of Sidebar sidebar to be first.

I had an impressions that widget should move to that special “unassigned” area in this situation, but maybe that only works between themes, not changes in same theme.