global.php

function myplugin_activate()
{
    global $wpdb;

    $q = "UPDATE {$wpdb->comments} SET user_id = '0'";
    $wpdb->query( $q );

    $query = "SELECT DISTINCT $wpdb->comments.comment_author, wp_users.ID FROM $wpdb->comments INNER JOIN wp_users ON ($wpdb->comments.comment_author = wp_users.display_name)";
    $comments_with_user = $wpdb->get_results( $query,  ARRAY_A );
    foreach( $comments_with_user as $key => $value )
    {
        $q = "UPDATE {$wpdb->comments} SET user_id = {$value['ID']} WHERE comment_author = '{$value['comment_author']}'";
        $wpdb->query( $q );
    }

    return TRUE;
}