Upgrade SQL Server 2014 Failover Cluster to SQL Server 2016 Availability Group on Windows Server 2016
To migrate from two Failover Clustered Instances (FCI) to an Availability Group (AG) that spans four Availability Replicas is fairly straight forward but somewhat of a lengthy process. Also, some downtime is required to point your application to the Availability Group Lister and manually fail over to one of the Secondary Replicas.

This is a high level overview of the process so use the links provided to plan:


Disable Database Mirroring
Completely uninstall the FCI and destroy Windows Cluster at DC2


Uninstalling a SQL Server Clustered Instance

Install Standalone SQL Instances on the two nodes at DC2
Join the two new stand-alone servers at DC2 to Cluster at DC1 and configure the AG


Combine a failover cluster instance and Availability Groups
Configure Availability Group on a multisubnet cluster

Take down time to fail over to to a Secondary Replica in DC2 and update Application to point to Listener
Remove FCI from AG


Remove a secondary replica from an Always On Availability Group

Completely uninstall the FCI at DC1
Repeat Step 3 at DC1
Add new stand-alone instances to AG


Add a secondary replica to an Always On Availability Group



All of this can be done on SQL Server 2014. There are some benefits to upgrading to 2016 but I would recommend you read this to help plan as there are a lot of similar steps to upgrade.