Domain upgrade is the process of upgrading the domain from one version of Active Directory to another by upgrading the operating system. The operating system itself is upgraded, but the forest and domain functional levels still must be upgraded thereafter. The disk housing the Active Directory database (NTIS.DIT) is recommended to have at least 20 percent free disk space before beginning the operating system upgrade. Note that this process is distinct from a domain migration.
Performing a domain upgrade involves two primary methods: an in-place upgrade and a swing server upgrade. In the in-place upgrade, you upgrade the operating system on the actual running domain controllers. However, a more common scenario is the swing server upgrade, in which you add a Windows Server 2012 server to the domain as a domain controller and retire one of the legacy domain controllers. This process would continue until all domain controllers are replaced, after which time the forest functional level could be upgraded.
Domain upgrade
