<?php
/*
Plugin Name: Update Comment Author
Plugin URI: http://qvister.se
Description: Make sure your previous comments gets updated when you change your display name.
Version: 0.1
Author: Anton Lindqvist
Author URI: http://qvister.se
*/

function update_comment_author($user_id, $user) {
    global $wpdb;
    $display_name_new = (isset($_POST['display_name'])) ? $_POST['display_name'] : NULL;
    $display_name_old = (isset($user->display_name)) ? $user->display_name : NULL;

    if (isset($display_name_new) && isset($display_name_old) && $display_name_new != $display_name_old) {
        $wpdb->update(
            $wpdb->comments,
            array('comment_author' => $display_name_new),
            array('comment_author' => $display_name_old),
            array('%s'),
            array('%s')
        );
    }

    return TRUE;
}
add_action('profile_update', 'update_comment_author', 1, 2);
?>