LCS and OCS Servers are not designed for
co-existence for longer time because of following
limitations.
Archiving interoperability is not supported.
Although you can use the Live Communications Server
2005 administrative snap-in with the Office
Communications Server 2007 administrative snap-in,
not all tasks can be done from both snap-ins, and
both snap-ins cannot be installed on the same
computer
Users who are enabled for enhanced presence can no
longer use Microsoft Office Communicator 2005 or
access specific components of Live Communications
Server 2005. Additionally these users can no longer
use the 2005 versions of Communicator Web Access or
Communicator Mobile.
Office Communications Server 2007 audio/video (A/V)
and Web conferencing features are not available for
users hosted Live Communications Server 2005 or any
users using Office Communicator 2005.