Hello MarkMillerADMIN-1204,
Thank you for posting in our Q&A forum.
As I understand, you have multiple 2012 R2 DCs (also ADFS servers), and you want to upgrade them from 2012 R2 to 2016, now you are upgrading 1st primary DC from Win Server 2012R2 to 2016.
From the following links, we can see, it seems to be not related to forest domain functional level, it is related to farm behavior level.
Administrators can add new Windows Server 2016 federation servers to an existing Windows Server 2012 R2 farm. As a result, the farm is in "mixed mode" and operates at the same farm behavior level as the original farm. To ensure consistent behavior across the farm, features of the newer Windows Server AD FS versions cannot be configured or used.
For more information, we can read articles below.
Upgrading to AD FS in Windows Server 2016 using a WID database
https://learn.microsoft.com/en-us/windows-server/identity/ad-fs/deployment/upgrading-to-ad-fs-in-windows-server
Upgrading to AD FS in Windows Server 2016 with SQL Server
https://learn.microsoft.com/en-us/windows-server/identity/ad-fs/deployment/upgrading-to-ad-fs-in-windows-server-sql
Hope the information above is helpful.
Best Regards,
Daisy Zhou
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