Which Windows Server 2012 license can I use with active Software Assurance after the end-of-life of Windows Server 2008 R2 Enterprise?
If your Windows Server 2008 R2 Enterprise licenses were coverd by active Software Assurance at the time of Windows Server 2012 release, Windows Server 2008 R2 Enterprise licenses can be exchanged into Windows Server 2012 Standard licenses. In addition to exchanging the licenses, it is also possible to perform a step-up on Windows Server 2012 Datacenter Edition.
Microsoft Product List Stand September 2012 (Page 164)
Whitepaper_Microsoft Windows Server 2012 Licensing FAQ