In my new project, I am involved in figuring out the migration of LCS 2005 to OCS 2007 R2.
Should you be in the same boat, you may appreciate the following list of prerequisites you will need on some/all of your LCS servers. And I suggest your read the supporting information on these updates and hotfixes. There are some stumbling points.
Ensure that Live Communications Server 2005 servers are patched with the following updates:
Live Communications Server Service Pack 1: This upgrade to the base LCS 2005 is available at http://www.microsoft.com/downloads/details.aspx?FamilyID=3508860C-2616-4B5A-BA00-353BE599A7B1&displaylang=en
Update for Live Communications Server 2005; February 10, 2006, available at http://support.microsoft.com/kb/911996
Live Communications Server 2005 post-Service Pack1 hotfix package: July 21, 2006, available at http://support.microsoft.com/kb/921543
Update for Live Communications Server 2005 Service Pack 1: September 2008 available at http://support.microsoft.com/kb/950614
Update your Communicator 2005 clients with the Communicator 2005 hotfix package: December 19, 2008 available at http://support.microsoft.com/kb/949280