For those who already have a deployment of Live Communications Server 2005 SP1 and are looking to migrate to Office Communications Server 2007, the migration is straightforward. However, there are a few considerations to keep in mind:
The impact of migrating users from legacy presence to the enhanced presence model
Upgrading users' Communicator 2005 client to Communicator 2007
The possible need to decentralize your deployment to support users in remote offices to do Web conferencing and VoIP
In addition, you need to consider the following issues applicable to any Office Communications Server 2007 deployment:
Scalability considerations for the new modalities: ...