Exchange CU10 update fails; Forest Functional Level needs to be raised; FF Level at 2012

Problem:CU 10 will not install because Forest Functional Level must be raised from 2008 R2.

We had 3 servers: 2 of them Server 2012 and 1 Server 2008 R2.

The 2008 server besides having AD also had Exchange 2010 from which we recently migrated to the new Exchange 2016 server.

I ran DCPROMO on the 2008 Server and removed AD; also removed AD server role. Turned off the 2008 server.

I then checked AD Domains and Trusts and ADSI Edit. Only the 2 current 2012 servers are listed under domain controllers. Raised Forest Functional Level to 2012. Waited an hour, CU10 Update still fails because it says that the FF Level needs to be raised. Waited 24 hours, same error.
CocosDadAsked:
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Michael B. SmithExchange & Active Directory ExpertCommented:
2008R2 FFL/DFL is sufficient for Exchange 2016.

Run DCDIAG. See what it says about replication. I'm guessing you've got some errors in there.
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Saif ShaikhServer engineer Commented:
Run the repadmin /syncall/AdeP from elevated prompt.

Also run following commands:
Check properly your domain with this command-line(PowerShell)
Import-Module ActiveDirectory

Get-ADForest

You can see your correct ForestMode (maybe windows2003Forest)

Raise now your Forest Functional Level (Run your PowerShell as Administrator)
Set-ADForestMode –Identity NAMEofTheforest.local -ForestMode Windows2008R2Forest
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