SQL Server 2008 mirroring migration to SQL Server 2012 always on
It really depends on your downtime window.

My preferred steps would be :


Create a full backup of the SQL Server 2008 R2 database and restore it on the SQL Server 2012.
Perform all of the post-restore steps.
Add the database to an Availability Group.


With modifications as per your environment, you can probably automatically add databases using T-SQL script.