As a first step to upgrade to Lync 2013 environment it’s better to push Lync 2013 client to all user’s computers and enable Lync 2013 to see OCS 2007 R2 servers.
It’s as simple as adding an registry key. Disabling Server version check on the registry makes Lync 2013 client to see the OCS 2007 R2 Front-end servers. Add the following registry key and reboot the computer.
Value: 1 (00000001)
You may download this file (right click the downloaded file and select Merge to add to the registry).
Of course there is always a catch. Lync 2013 client with OCS servers works only with basic instant messaging (no audio/video, screen sharing, etc.,). One good thing is if you use Outlook 2013, Lync 2013 client can save IM history in Exchange Mailbox.