I currently have 200 category pages and a url structure like this: website.com/category-name but now I need to add parent categories in order to organize all the categories.

The problem I have is that all the URL:s in the sitemap will change, which will have a deadly impact on my traffic from a SEO perspective.

I use the following code to remove the parent category from the URL so the URL isn’t as follows: website.com/parent-name/category-name/ but this does not change the url in the sitemap of course. How would you solve this predicament?

    // Add our custom category rewrite rules
add_filter('category_rewrite_rules', 'no_category_parents_rewrite_rules');
function no_category_parents_rewrite_rules($category_rewrite) {
    //print_r($category_rewrite); // For Debugging

    $category_rewrite=array();
    $categories=get_categories(array('hide_empty'=>false));
    foreach($categories as $category) {
        $category_nicename = $category->slug;
        $category_rewrite['('.$category_nicename.')/(?:feed/)?(feed|rdf|rss|rss2|atom)/?$'] = 'index.php?category_name=$matches[1]&feed=$matches[2]';
        $category_rewrite['('.$category_nicename.')/page/?([0-9]{1,})/?$'] = 'index.php?category_name=$matches[1]&paged=$matches[2]';
        $category_rewrite['('.$category_nicename.')/?$'] = 'index.php?category_name=$matches[1]';
    }
    // Redirect support from Old Category Base
    global $wp_rewrite;
    $old_base = $wp_rewrite->get_category_permastruct();
    $old_base = str_replace( '%category%', '(.+)', $old_base );
    $old_base = trim($old_base, "https://wordpress.stackexchange.com/");
    $category_rewrite[$old_base.'$'] = 'index.php?category_redirect=$matches[1]';

    //print_r($category_rewrite); // For Debugging
    return $category_rewrite;
}

// Add 'category_redirect' query variable
add_filter('query_vars', 'no_category_parents_query_vars');
function no_category_parents_query_vars($public_query_vars) {
    $public_query_vars[] = 'category_redirect';
    return $public_query_vars;
}
// Redirect if 'category_redirect' is set
add_filter('request', 'no_category_parents_request');
function no_category_parents_request($query_vars) {
    //print_r($query_vars); // For Debugging
    if(isset($query_vars['category_redirect'])) {
        $catlink = trailingslashit(get_option( 'home' )) . user_trailingslashit( $query_vars['category_redirect'], 'category' );
        status_header(301);
        header("Location: $catlink");
        exit();
    }
    return $query_vars;
}