A generally accepted best practice for migrations is to create several test users and use them to conduct systems tests. After you have successfully moved and tested those accounts, you should identify a group of pilot production users and move their accounts and conduct validation tests on them. If the results are satisfactory, you can move the rest of your users to the new deployment.
Consider your compliance requirements before you move user accounts. Office Communications Server 2007, Archiving Server, cannot archive traffic from users who are homed on a Live Communications Server pool, and the Live Communications Server Archiving service cannot archive traffic from users who are homed on a Office Communications Server 2007 R2 pool. If you have compliance requirements, you have to deploy Office Communications Server 2007 R2, Archiving Server, before moving users to the Office Communications Server 2007 R2 pool. In addition, if you intend to deploy the new Group Chat feature and achieve compliance with that feature set, you need to deploy Group Chat archiving. For details about Group Chat compliance, see in the Deploying Group Chat Server documentation.
Your existing environment dictates how your migration proceeds. If you currently have Live Communications Server Access Proxy in place to provide external (that is, Federation and Public IM Connectivity, also known as PIC) and remote user access, and you plan to continue to offer these services, you start your migration with the Office Communications Server 2007 R2 Edge Server and Director and then you implement the Office Communications Server 2007 R2 pool.
After you have migrated all of the server roles and moved all users to the new pool(s), you can deploy the new Office Communicator 2007 R2 client.