How to Fix 404 when changing WordPress permalinks

If you’ve updated your WordPress permalinks from
/postname/ → /category/postname/,
you may notice old links returning 404 errors. That’s because WordPress can’t automatically redirect visitors to the new category-based URLs.

Here’s a quick fix that keeps your SEO intact and sends users to the right place.

⚙️ Step-by-Step

Log in to your WordPress admin.

Go to Appearance → Theme File Editor (or access your site files via FTP).

Open your theme’s functions.php file.

Paste the code below at the bottom:

add_action('template_redirect', function() {
    if (is_404()) {
        global $wpdb;
        $request_uri = trim($_SERVER['REQUEST_URI'], '/');
        $slug = basename($request_uri);
        $post_id = $wpdb->get_var($wpdb->prepare(
            "SELECT ID FROM $wpdb->posts WHERE post_name = %s AND post_status = 'publish' LIMIT 1",
            $slug
        ));
        if ($post_id) {
            wp_redirect(get_permalink($post_id), 301);
            exit;
        }
    }
});

Now, any visitor who lands on an old /postname/ URL will be automatically redirected to the correct /category/postname/ version — no broken links, no SEO loss.

Simple, quick, and future-proof.