I've found plenty of articles for upgrading SQL in a clustered environment from SQL 2008 to SQL 2008 R2. However, I have not found much information in relation to migrating a current Server 2008 Failover Cluster running multiple SQL versions to a Server 2008 R2 Failover Cluster.
Since you can't join a 2008 R2 server to a 2008 failover cluster, what is the process for this?