Migrate to sql 2017 and add ag's A client wants to migrate from sql 2008 to sql 2017 and add availability groups. All enterprise. All vm's.

Question: (without using any words starting with Az~ smile) should I set up the ag's first or last?

My instinct is to (side by side) migrate as is. Then once live and stable, separately, add always-on ag's.
My instinct is to (side by side) migrate as is. Then once live and stable, separately, add always-on ag's.


That is perfectly reasonable and supported.

However.

Are you sure you really want to perform the cluster creation, configuration and AG provisioning and testing on a live (post-migration) system?  

If you do the AGs first, you can perform all that configuration and testing as many times as you want before the migration.