A Moderator with Can Edit Posts permission is allowed to change the display name if the post was posted by a guest even if the Moderator does not have Can Change Post Author permission. While I understand the difference between changing the actual user and the guest's display name, this technicality will escape most users and should be fixed.

HGN posted a simple two-edit fix for this here (http://www.phpbb.com/community/viewtopic.php?p=13144890#p13144890) – although changing SubSilver would also be required. I provided some analysis of why I think this change should be included in phpBB here (http://www.phpbb.com/community/viewtopic.php?p=13145127#p13145127).