I have a Windows Server 2008R2 DC, and I want to upgrade the operating system (on the same hardware).
From information on the internet, I have seen how to upgrade Active Directory, though my server also has other roles and features like DNS, Group Policies and File Services.
Will an AD and operating system upgrade from 2008R2 to 2012R2 also automatically upgrade Group Policies, DNS and File Services?
To keep thing simple, you should instate 2012 r2 member server in domain and upgrade AD from there promote it to ADC
Then transfer FSMO roles to new DC, decommission old DC, upgrade / fresh install OS to 2012 R2 and then promote it to 2012 R2 ADC