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After preparing and staging a Remote Desktop Services upgrade, you should be able to perform migrations for each RDS role installed on a server. While the actual steps involved can vary with your internal policies and preferences, the migration should follow the same general flow.
Migrating the RD Connection Broker servers to Windows Server 2012 R2 is an important first step. Tackling the Connection Broker migration first is recommended because RD Virtualization Host, Session Host and Web Access roles all depend on it. A typical migration downtime can often last up to three hours, and users cannot access RD resources (such as personal virtual desktops) during the process.
Next, move RD Session Host servers. It's recommended to upgrade half of the Session Host servers first by logging those sessions off, removing the affected servers from the collection, executing the migration to Windows Server 2012 R2, and then adding the upgraded servers back to the collection.
Migrate the remaining half of the Session Host servers separately. This helps to mitigate downtime by providing some level of RD Session Host support. Session Host migrations take about an hour, but RD resources, such as RemoteApp programs, won’t be available until they are installed on the new servers.
The next step should be the RD Virtualization Host servers. Microsoft recommends moving all Virtualization Host servers to Windows Server 2012 R2 at once. This involves logging users off, turning off and saving all virtual machines on each host, performing the upgrade, and then reconnecting the Virtualization Hosts after the migration is complete. The process can take several hours, depending on the number of VMs you migrate, but it can be significantly more involved for environments with multiple RD Virtualization Host servers.
The last role to migrate is RD Web Access, which will take about an hour. Any resources that depend on Web Access and are handled through Connection Broker will be unavailable. There are no significant dependences, so the remaining RD Licensing and Gateway roles can be migrated at any time.