function change_username_idc() {
    $old_username = 'old_username'; // Replace with the old username
    $new_username = 'new_username'; // Replace with the new username
    $new_nickname = 'default'; // Replace with the new nickname (set as 'default' to leave it unchanged)

    global $wpdb;

    $table_name = $wpdb->prefix . 'users';

    $user = $wpdb->get_row(
        $wpdb->prepare(
            "SELECT * FROM $table_name WHERE user_login = %s",
            $old_username
        )
    );

    if ($user instanceof stdClass) {
        $result = $wpdb->update(
            $table_name,
            array(
                'user_login' => $new_username,
                'display_name' => $new_username,
            ),
            array('ID' => $user->ID),
            array('%s', '%s'),
            array('%d')
        );

        if ($new_nickname !== 'default') {
            update_user_meta($user->ID, 'nickname', $new_nickname);
        }
    }
}

add_action('init', 'change_username_idc');