To upgrade all 6 nodes, it would be very similar to the single cluster upgrade.

NOTE: Upgrading all 6 nodes WILL STILL REQUIRE AN OUTAGE due to the fact that the upgrade of the first node will also upgrade the SQL DB at the same time…and all nodes not yet upgraded (including the 3 nodes in the secondary DC) will not be available while the DB is upgraded and the nodes are not upgraded.

Step 1. Take a backup of everything (Snapshots of all nodes, of SQL servers, SQL DB backups, etc.).

Step 2. Remove all nodes from the load balancer except the master node from the primary data center.

Step 3. Upgrade the primary DC node that is still connected to the load balancer.

Step 4. After the node is upgraded, leave it connected to the load balancer.

Step 5. Upgrade the other nodes one at a time, starting with the two remaining nodes in the primary data center.

Step 6. After each of the nodes are upgraded, add them back to their local load balancer.

NOTE: Once each one of the secondary DC nodes is upgraded, add them to their local load balancer to bring them online.