I don't think you can change the database owner of a mirror database. The mirror is in a non-recovered state so you can't run any statements against it.

You could change the owner of your primary database if that's an option or just leave it. As far as I remember it's not required that both databases have the same owner and the simple fact that to setup mirroring you have to restore a backup on the mirror server, will cause in most cases that the owners are different.

On my servers I always have "sa" as the owner of all databases, but when I restore a database initially I will become the owner, because we never logon as "sa".

Markus