There is one way to upgrade Failover cluster from Windows 2008 R2 to Windows 2012 R2:
1. Evict one of the nodes from the cluster, then do the clean installation, add all necessary futures;
- install StarWind
- create the new HA devices for new Windows 2012 R2 Cluster Node
- connect it in iSCSI initiator. Initialize this disks in Disk Management, as writes in this article:
https://www.starwindsoftware.com/resour ... 0e77312268
2. Create new failover cluster on the Server 2012R2, then, we have two one-node failover clusters, the old cluster, and new cluster;
3. Then cope roles(VM's) from the old cluster to the new cluster;
4. Ensure the new cluster works well, then, do a clean installation on the other node:
- start the replication process from node 1 to node 2 in the StarWind management console
- connect this disks in iSCSI initiator on node 2
https://www.starwindsoftware.com/resour ... 0e77312268
5. Add to the new cluster.
Cluster in-place upgrade it not support, we need to use two clusters, the old one and the new one, then cope roles and migrate storage between the two clusters.
In summary, you need downtime. Sorry.