Here are the components: WSFC Name: SQLPROD1 Existing Nodes in this "cluster": SQL2014OnSite and SQL2014OffSite New Nodes to be added (?) to SQLPROD1: SQL2016OnSite and SQL2016OffSite All 4 servers running Windows Server 2012 R2 Enterprise. All 4 instances of SQL Server are Enterprise Edition.
I successfully added the two new servers to the WSFC. I successfully configured the two new SQL2016 instances to use AlwaysOn.
For testing purposes, I created a new AOAG named TestMigrate with a single database, AdventureWorks2014 and configured it to be identical to the 3 production AOAGs (Asynchronous commit, manual failover and non-readable secondary).
I then proceeded to add the 2 new nodes as replicas to the AG (worked great). I read this article (https://msdn.microsoft.com/en-us/library/dn178483.aspx), in the section titled "Availability Group with One Remote Secondary Replica" that indicated it was necessary to change the Availability Mode to Synchronous Commit, then failover to the new primary node, and finally change the Availability Mode back to Asynchronous Commit. Up to this point, everything worked as expected.
The problem happened when I removed the original 2 nodes from the AG. Within a few seconds, both replica copies of the database switched to "Restoring", and the AG disappeared from both 2016 instances. Is there some "background" metadata that links the AG to the instance where it was created?
Also, it was my assumption that this was the best way to upgrade/migrate these AGs, but since it completely dropped the AG once I removed the two original nodes/instances, I'm guessing it's not...or I have missed a rather crucial step. I've searched high and low online for a straight-forward approach to accomplish this task, but haven't found anything that addresses this specific requirement. Several "similar" tasks, but not quite the same...and not close enough to give me the "missing piece(s)".
Any suggestions or recommendations would be very much appreciated.