If you are planning to upgrade 2012 DC server to 2016 withouth taking in consideration reinstallation scenario you need to be aware about problems that you will face about netlogon and Network Time servicing.
Here they are related Microsoft articles:
Windows Time Service settings are not preserved during an in-place upgrade to Windows Server 2016 or Windows 10 Version 1607
https://support.microsoft.com/en-my/help/3201265/windows-time-service-settings-are-not-preserved-during-an-in-place-upgrade-to-windows-server-2016-or-windows-10-version-1607
Netlogon service doesn't retain settings after upgrade to Windows Server 2016
https://support.microsoft.com/sl-si/help/3201247/netlogon-service-doesn-t-retain-settings-after-upgrade-to-windows-server-2016
Here it is an article that can be useful for reading purpose:
https://www.devadmin.it/2017/05/08/issues-durante-upgrade-in-place-di-domain-controller-a-windows-server-2016/
