Hyper-V How To: Upgrade Hyper-V to Hyper-V R2
KB957256 describes the options that are available for upgrading or migrating from a Windows Server 2008 installation with the Hyper-V role enabled to Windows Server 2008 R2. For Windows Server 2008 Failover Clusters that are running virtual machines, see the "Upgrade guidance for virtual machines on failover clusters" section of the KB.
See also
http://technet.microsoft.com/en-us/library/cc730990.aspx
http://blogs.msdn.com/clustering/archive/2009/09/02/9890272.aspx
