Remote Desktop Services (Terminal Services) Team Blog : Windows Server 2008 R2 RDS and Windows Server 2008 TS CAL Compatibility:
If it wasn’t bad enough figuring out licensing…. now TS cals are no longer TS cals but they are R2RDS cals in the Windows 2008 R2 era. I think I’ll be calling it TS for a long long time.
“Please note that in order to install Windows Server 2008 R2 CALs on your Windows Server 2008 license server, you need to request your Technical Account Manager (TAM) or Escalation Engineer (EE) for KB 971302 and install it on the license server.”
Or they can also use the trick where you edit the request hotfix page and get the request url directly: http://support.microsoft.com/hotfix/KBHotfix.aspx?kbnum=971302
Keep in mind that Windows 2008 R2 cannot be installed over the top of SBS 2008 as well (yes we tried, we wanted to see if it would be blocked or if things would blow up)
So no, you can’t do an inplace upgrade of SBS 2008 to and R2 era build of Windows 2008 to get that direct access/branch cache stuff.