A few weeks ago I had a reminder of one of the finer points of the requirements for mirroring: mirrored servers need to be running not only the same version of SQL Server, but the same edition as well.
I hadn’t thought of this in a while, but it makes sense. Asynchronous Database Mirroring (also known as “High-Performance Mode”) is only available in Enterprise Edition, while Standard Edition only supports “High Safety Mode”, which is synchronous. If the primary server in a mirroring topology was Enterprise Edition, but the mirror server was Standard Edition, how would that work?
Also check Glenn Berry’s comment for a trick which stretches mirroring’s capabilities for upgrades.