Q:

AG Cluster db upgrade by vendor

Vendor wants to put db into simple mode prior to upgrade then back into full recovery mode after upgrade. I do not want to do this because I am not sure what impact it will have on the db cluster. Won't have to drop the db from the AG then put it back in after this process?

A:

As seen in the availability group prerequisites the database must be in full recovery mode.
To change the recovery model, the database must be removed from the Availability Group, and it must be changed back to FULL before adding back into the Availability Group.
Obviously be very careful here - changing the database to single mode will break your log backup chains so you will be without that recovery mechanism if it goes wrong.

