Microsoft Windows Server 2012 Upgrade Path

Domain controllers that run 64-bit versions of Windows Server 2008 or Windows Server 2008 R2 can be upgraded to Windows Server 2012.

You cannot upgrade domain controllers that runs on Windows Server 2003, 32-bit versions of Windows Server 2008. To replace them, install domain controllers that run Windows Server 2012 in the domain, and then remove the domain controllers that are running on Windows Server 2003. Bottom line is you cannot upgrade or migrate your Windows Server 2003 to Windows Server 2012. But I am not talking about third party tools here.

Editions you could be running currently

Upgrade possibilities

Windows Server 2008 Standard with   SP2ORWindows Server 2008 Enterprise   with SP2 Windows Server 2012 StandardORWindows Server 2012 Datacenter
Windows Server 2008 Datacenter   with SP2 Windows Server 2012 Datacenter
Windows Web Server 2008 Windows Server 2012 Standard
Windows Server 2008 R2 Standard   with SP1ORWindows Server 2008 R2 Enterprise   with SP1 Windows Server 2012 StandardORWindows Server 2012 Datacenter
Windows Server 2008 R2 Datacenter   with SP1 Windows Server 2012 Datacenter
Windows Web Server 2008 R2 Windows Server 2012 Standard

For more information about supported upgrade paths, see Evaluation Versions and Upgrade Options for Windows Server 2012.

Please be mindful that you cannot convert a domain controller that runs an evaluation version of Windows Server 2012 directly to a retail version.

Due to a known issue, you cannot upgrade a domain controller that runs a Server Core installation of Windows Server 2008 R2 to a Server Core installation of Windows Server 2012. The upgrade will hang on a solid black screen late in the upgrade process. Rebooting such DCs exposes an option in boot.ini file to roll back to the previous operating system version. An additional reboot triggers the automatic rollback to the previous operating system version. Until a solution is available, it is recommended that you install a new domain controller running a Server Core installation of Windows Server 2012 instead of in-place upgrading an existing domain controller that runs a Server Core installation of Windows Server 2008 R2. For more information, see KB article 2734222.