I don't know if it was mentioned before, but few days ago I noticed that, when visiting profile of user that changed username, there is no note in security moderators log.
To be honest im not sure if Security moderators log would be good place to have a note like that, but it would certainly be good thing to document username changes. Specially in case if someone takes old username of person who changed it.
The general consensus had been that we shouldn't really be tracking username changes since the top reason for username changes have typically been done for privacy reasons, so we try to respect that.
Here was theymos' response to this in the past. So, we've opt'd to not track that.