Create membership number for existing/new users

I have a membership site with 4000+ users currently on it.

I am needing to give both new/existing users an automatic ID number.

its very similar to this: Create Unique ID for user

However, this only works for new users, I need it to work for the current users too. So for instance, user id 3421 would get membership number 3421.

This is what I have so far: (pretty much all from above link)

function fb_add_custom_user_profile_fields( $user ) { ?
        h3?php _e('Extra Profile Information', 'Avada'); ?/h3
            table class=form-table
                tr
                    th
                        label for=memnumber?php _e('Membership Number', 'Avada'); ?
                        /label
                    /th
                    td
                    input type=text name=memnumber id=memnumber value=?php echo esc_attr( get_the_author_meta( 'memnumber', $user-ID ) ); ? class=regular-text /br /
                    /td
                /tr
            /table
        ?php }

    function fb_save_custom_user_profile_fields( $user_id ) {
        if ( !current_user_can( 'edit_user', $user_id ) )
            return FALSE;
        update_usermeta( $user_id, 'memnumber', $_POST['memnumber'] );
    }
    add_action( 'show_user_profile', 'fb_add_custom_user_profile_fields' );
    add_action( 'edit_user_profile', 'fb_add_custom_user_profile_fields' );
    add_action( 'personal_options_update', 'fb_save_custom_user_profile_fields' );
    add_action( 'edit_user_profile_update', 'fb_save_custom_user_profile_fields' );




    add_action( 'user_register', 'assignuserid');

    function assignuserid($user_id) {
    global $wpdb;
    $latestid=$wpdb-get_var(SELECT meta_value from $wpdb-usermeta where meta_key='memnumber' order by meta_value DESC limit 1;);
    update_user_meta( $user_id, 'memnumber', $latestid +1 );
}

Any advice would be appreciated.