I already covered adding Windows Server 2012 as DC to Windows Server 2008 AD environment here –https://www.informaticar.net/?p=1784 . If you want to get rid of Windows Server 2008 DCs from your network, your first step should be transferring FSMO roles from Windows Server 2008 to Windows Server 2012 DC. Read More
Demote Windows Server 2008 DC: Transfer FSMO Roles
j j j