Have you ever thought about this? Can we Mirror a Database from SQL Server 2008SP2(Principal) to SQL Server 2008(Mirror) RTM? Well, how about Mirroring from SQL Server 2005 to SQL Server 2008?? Let me show you the prior case.
Principal DB Engine Version: SQL Server 2008 SP2
Mirror DB Engine Version: SQL Server 2008 RTM
DB Name: DB_Mirror
Answer: Yes, we can! You can see below screenshot.
You should not see any Issues with Failover/Switching DB Roles from Principal to Mirror and Viceversa.
Principal DB Engine Version: SQL Server 2005 SP3
Mirror DB Engine Version: SQL Server 2008SP2/2008R2.
I don’t have a SQL Server 2005 Instance to show you how it works, but Yes, we can! Very Important point to consider in this case is, Once you failover(Switch the Mirroring Roles) the Database from SQL 2005 to 2008, you can’t failback! This is One-Way approach.
Note: You can’t setup Mirroring from 2008(as Principal) to 2005(as Mirror). Well, DB Version can’t be downgraded. In other words, we can’t restore a DB to lower Versions!!
Hope this is Informative and something which you can think about as an option for upgrading/Migrating Large Databases with very less acceptable downtime! ( Make sure, you understand all the caveats before trying this in your Prod Environments).