I've tried a lot of different things with extending that datawriter, had success with none but what ragtek suggested with init_dependencies a month ago. Not very compelled to go play with/change that again but if someone has success with it, I'm curious to know.
In XenForo 1.2 this was moved from the User DW to the Usermodel.
* Updates existing content with a new username, or updated user ID
* @param integer $existingUserId
* @param string $newUserName
* @param string $oldUserName
* @param integer $newUserId (set if the user ownership has changed)
public function changeContentUser($existingUserId, $newUserName = null, $oldUserName = null, $newUserId = null)