Se todos os controladores de domínio forem Windows Server 2012, então sim, é seguro elevar o nível funcional do Domínio para o Windows Server 2012. Aqui estão algumas diretrizes da Microsoft sobre níveis funcionais de domínio e floresta:
You can raise the functional level of a domain only if all domain
controllers in the domain run the version or versions of Windows
Server that the new functional level supports.
You can raise the functional level of a forest only if all domain
controllers in the forest run the version or versions of Windows
Server that the new functional level supports.
You cannot set the domain functional level to a value that is lower
than the forest functional level, but you can set it to a value that
is equal to or higher than the forest functional level.