Let’s say you’re upgrading your OCS environment to Lync 2010 and you want to install Lync client on your work stations prior to moving your users to the new Lync 2010 pool.
If you’ll try to connect Lync 2010 client to OCS 2007 you’ll get an error, even after you enabled the OC version in OCS.
To bypass that, you’ll have to disable the Lync client server check.
Copy the following text to your text editor and save it as a .reg file, then double-click to update to registry:
Windows Registry Editor Version 5.00
You can also run the following command:
reg add HKCU\Software\Policies\Microsoft\Communicator /v DisableServerCheck /t REG_DWORD /d 1 /f
Now you can easily sign in to OCS using your Lync client.