Changing an alias is done by sending a request to Admin to have it changed to whatever new alias you want. It is frequently done by many people over the Christmas holidays to a more seasonally appropriate alias and then changed back afterwards. When you do change your alias, all posts with the old alias are changed to the new one. So clearly there is no attempt to "cover his tracks". Seems to me that NOT changing the old posts "would screw with the historical record" much more than having all posts reflect the current alias.