AD DS improvements apply to its forest and domain functional levels. Upgrading the operating system or adding domain controllers that run Windows Server 2016 to an existing AD infrastructure isn't going to upgrade the forest and domain functional levels. In order to use or test these new AD DS 2016 features, you need to have the forest and domain function levels set to Windows Server 2016. The minimum forest and domain functional levels you can run on your identity infrastructure depend on the lowest domain controller version running.
For example, if you have a Windows Server 2008 domain controller in your infrastructure, even if you add a Windows Server 2016 domain controller, the domain and forest functional levels need to be maintained as Windows Server 2008 until the last Windows Server 2008 domain controller is removed from the infrastructure.
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