In nearly every migration, you should use the side-by-side migration path. In a side-by-side migration, you deploy a new server with Lync Server 2010 alongside a corresponding server that is running Office Communications Server 2007 R2, and then you transfer operations to the new server. If it becomes necessary to roll back to Office Communications Server 2007 R2, you have only to shift operations back to the original servers. Be aware that in this situation any new meetings scheduled with upgraded clients will not work, and the clients would also need to be downgraded.

After you have deployed Lync Server 2010 in parallel with Office Communications Server 2007 R2, the topology represents a coexistence testing state of the two deployments. Before migrating to Lync Server 2010, it is important to test both deployments and ensure services are started; each site can be administered, and clients can communicate with current and legacy users. Prior to migrating all users in Phase 8: Move from Pilot Deployment into Production, it is very important that you understand the state of each deployment and ensure that each deployment is functional and working properly. Typically, this coexistence testing phase exists throughout the pilot testing of Lync Server 2010. Legacy users are moved to Lync Server 2010 for a period of time to ensure that application compatibility and features and functions are working properly. After pilot testing, users and applications are moved to the production version of Lync Server 2010, and the legacy pools and applications of Office Communications Server 2007 R2 are retired.

The recommended method for migrating to Lync Server 2010 is described in Phase 4: Merge Topologies. This method uses the Topology Builder merge wizard feature. An alternate method, described in Migrate Using Lync Server Management Shell (optional), uses a command-line approach to migrating. This method requires creating an input file with Edge Server information, and running a series of Lync Server Management Shell commands. While this method replaces the Topology Builder Merge wizard procedure described in Phase 4: Merge Topologies, the recommended process is to use Topology Builder and follow the guidelines in Phase 4. However, if you do plan to use this command-line based approach, read the topic thoroughly.