Windows 10 will be available on July 29th 2015. Microsoft has prepared for this by already making an Update(KB3058168) that enables Windows 8.1, Windows Server 2012 R2, Windows 8, and Windows Server 2012 Key Management Service (KMS) hosts to activate a “later version of Windows”. This must means Windows 10. I do not know if this means that is even for Windows Server 2016. Windows 10 will be activated by a KMS server running this update but it might.
Select the version you need for the KMS server or servers you use and install them.