Rolling cluster upgrade
Since Windows Server 2016, there is a new way to migrate a Windows Server 2012 R2 cluster to Windows Server 2016. This is called Rolling cluster upgrade. Now you can mix in the same cluster nodes running on Windows Server 2012 R2 and Windows Server 2016.
Microsoft has implemented a cluster functional level. A cluster running on Windows Server 2012 R2 is at cluster version 8 while a cluster running on Server 2016 is at cluster version 9. When a cluster contains Windows Server 2012 R2 and Windows Server 2016 nodes, it is in mixed OS mode but still in cluster version 8.
To upgrade the cluster, you have to evict each node one by one. When a node is evicted, you have to upgrade the node to Windows Server 2016 (I recommend that you do a clean install). Once the node is in Windows Server 2016, you can add it again to the cluster:
Rolling Cluster Upgrade
Once all cluster nodes are upgraded to Windows Server 2016, you can run this PowerShell cmdlet:
Update-ClusterFunctionalLevel...