As customers were beginning to upgrade OCS 2007 R2 deployments with Lync, the question of support for Group Chat showed that what was originally thought to be a simple answer required a more complex matrix to illustrate.
After conferring with the appropriate teams, this is the verbiage and table we submitted for inclusion in the document refresh.
August 2 - Technet now contains this information and as such is considered the authoritative information. Any conflict or confusion please alert me so I can rectify. http://technet.microsoft.com/en-us/library/gg615442.aspx
Supported Client and Server Coexistence
Customers migrating from Office Communications Server 2007 R2 to Microsoft Lync Server 2010 along with Group Chat can coexist under specific combinations of Server versions and Client versions. The following table summarizes the multiple combinations indicating support or not. We do not support the 2010 clients connecting to an R2 server.
OCS 2007 R2 / Group Chat Server R2
OCS 2007 R2 / Group Chat Server 2010
Lync Server 2010 / Group Chat Server R2
Lync Server 2010 /
Group Chat Server 2010
Communicator 2007 R2 / Group Chat Client 2007 R2
Communicator 2007 R2 / Group Chat Client 2010
Lync 2010 / Group Chat 2007 R2
Lync 2010 / Group Chat 2010