UPNs changed, login issues

Windows 2012 AD Environment

In order to prep for O365, we recently change everyones UPN to match their PrimarySMTPAddress.
Not changing their User Logon Name (pre-Windows 2000), that stayed the same.

About 90% of the users are still able to log in using their Pre-Windows 2000 logon names and/or new UPN (email address), but there are reports coming in that many can not log in.

Anyone know what I could be looking for to troubleshoot? Thinking it could be AD replication perhaps? depending what global catalog server they may be hitting?

Any ideas?
Christian HansUndecided... Asked:
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Will SzymkowskiSenior Solution ArchitectCommented:
A should be able to login successfully using sAMAccountName or UPN (does not matter). However you will need to reference the Security Logs on the Domain Controllers. For this you should also have Active Directory Auditing Enabled to ensure that you are getting all of the necessary communication.

I have created a HowTo for setting up AD Auditing (see below link).

Also checking replication and DC health use the following commands...
- repadmin /replsum
- repadmin /showrepl
- repadmin /bridgeheads
- DCDiag /v


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