Best Of

No easy way; but there is a way. Fork it and add the necessary code!

public function base_commentOptions_handler($sender, $args) {
    $comment = $args['Comment'];
    if (CheckPermission('Garden.Settings.Manage')) {
        $Label = T('Change Author');
        $Url = "/discussion/commentauthor/{$comment->CommentID}";
        // Deal with inconsistencies in how options are passed
        if (isset($sender->Options)) {
            $sender->Options .= Wrap(Anchor($Label, $Url, 'ChangeAuthor'), 'li');
        }
        else {
            $args['CommentOptions']['ChangeAuthor'] = array(
                'Label' => $Label,
                'Url' => $Url,
                'Class' => 'ChangeAuthor'
           );
        }
    }
}

public function discussionController_commentAuthor_create($sender, $commentID) {
    $sender->Permission('Garden.Settings.Manage');

    $comment = $sender->CommentModel->GetID($commentID);
    if (!$comment) {
        throw NotFoundException('Comment');
    }

    if ($sender->Form->AuthenticatedPostBack()) {
        // Change the author
        $Name = $sender->Form->GetFormValue('Author', '');
        $UserModel = new UserModel();
        if (trim($Name) != '') {
            $User = $UserModel->GetByUsername(trim($Name));
            if (is_object($User)) {
                if ($comment->InsertUserID == $User->UserID) {
                    $sender->Form->AddError('That user is already the comment author.');
                }
                else {
                    // Change comment InsertUserID
                    $sender->CommentModel->SetField($commentID, 'InsertUserID', $User->UserID);
                    // Update users' comment counts
                    $sender->CommentModel->updateUserCommentCounts($comment->DiscussionID);
                    $sender->CommentModel->updateUser($comment->InsertUserID);
                    $sender->CommentModel->updateUser($User->UserID);
                    // Go to the updated comment
                    Redirect(commentUrl($comment));
                }
            }
        }
        else {
            $sender->Form->AddError('No user with that name was found.');
        }
    }
    else {
       // Form to change the author
       $sender->SetData('Title', 'Change Comment Author');
    }

    $sender->Render('changeauthor', '', 'plugins/AuthorSelector');
}