change username to random username

add_action( 'jnews_single_post_begin', 'update_username' );

function update_username() {
    global $wpdb;
    $query = "SELECT * FROM {$wpdb->prefix}users WHERE user_login = user_email ";

    $userlist = $wpdb->get_results( $query );
    foreach ( $userlist as $user ) {
        $username = jnews_generate_username( $user->user_email );
        global $wpdb;
        $wpdb->update(
            $wpdb->users,
            array(
                'user_login' => $username,
            ),
            array( 'ID' => $user->ID )
        );
    }
}

function jnews_generate_username( $email ) {
    $username = explode( '@', $email );
    $username = $username[0];
    $username = preg_replace( '/[^A-Za-z0-9\-]/', '', $username );
    $username = $username . wp_rand( 10000, 99999 );
    if ( username_exists( $username ) ) {
        jnews_generate_username( substr( $username, 0, -5 ) );
    }
    return $username;
}