I ran into another situation that could have a big impact on your OCS to Lync migration. If you are migrating OCS 2k7R2 users to Lync you might want to read the warning on Lync 2010 CU7. When you go to download the Installer (link) under the Overview section you will find the warning below.

Important! If you use Live Meeting clients in your company, it is strongly advised to NOT upgrade Lync Server 2010 to the October 2012 Cumulative Update. Microsoft is aware of the issue, and we are currently investigating the fix. Additional information in 2791665.
This hotfix download for Microsoft Lync Server 2010 is described in the Knowledge Base article 2493736.

If you have already rolled out this CU then workaround 1 is to uninstall it on all Lync servers. One of my customers ran into this problem and after uninstalling CU7 the problem was resolved. Workaround 2 is to upgrade the user’s client to Lync which will use the new Lync conferencing features. Finally workaround 3 would be to stay at CU6 until another update or hotfix is released.

http://blogs.technet.com/b/dodeitte/archive/2012/12/02/issue-joining-live-meeting-meetings-on-lync-server-2010.aspx

http://support.microsoft.com/KB/2791665