We currently are updating all our Active Directory usernames from the old format (FirstnameLastinitial) to a new format (FirstnameLastname). So far, the Windows side of these changes have been working great, no issues with the change. When it comes to changing the OSX machines, things start to break.
On AD, we changed the username, both the "User logon name" and the "User logon name (pre-Windows 2000)", to the new format. This breaks the on-network logon on our Macs, and that user cannot log in at all, with both the old and new username. If i change the username back, after replication is done I can log back in.
So far my searching has only turned up mainly issues with AD passwords, not the username. I did find the ADacctChange script from https://clburlison.com/ADacctChange/ however the script would not work on my test Mac (both 10.10.5 and 10.11.4).