Quickest way to migrate mirroring to AG
If you want to ensure you minimize the amount of time you are without HA during your migration, here's a trick I recently used:


Set up the AG with a "dummy" database so that your AG is ready in advance. 
Log ship the database(s) to both your primary &amp; secondary AG replicas--at this point, the log shipped database is not part of the AG, you're just doing the log shipping in parallel to all nodes.
When you're ready to cut-over:


Take your final tail log backup on the old server.
Restore the tail log to both AG replicas, and on the primary node, restore WITH RECOVERY. 
Take a log backup from the primary node, and restore it to secondary WITH NORECOVERY. (At this point, your secondary node is pre-seeded and ready to be added to the AG)
Add the DB to the AG, without having to do backup/restore.