Q. Does the Windows Server 2016 rolling cluster upgrade only work for Hyper-V clusters?
A. Windows Server 2016 introduced the ability to have a cluster with a mix of Windows Server 2012 R2 and Windows Server 2016 nodes. This is not designed as a long term deployment but rather to facilitate the upgrade from Windows Server 2012 R2 to Windows Server 2016. With this mix capability it is possible to evict a 2012 R2 node from the cluster, reinstall it with Windows Server 2016 then add back into the existing cluster. This would be repeated for each node in the cluster until all nodes are running Windows Server 2016 at which point the cluster is switched to Windows Server 2016 mode to light up new 2016 functionality.
While this functionality is often highlighted for Hyper-V clusters there is nothing Hyper-V specific about it and will work for any 2012 R2 cluster being moved to 2016.