It is recommended to perform these actions in a V.Center environment:
A. Create 3 new Virtual Machines from the latest released (main) version. Do not launch them yet.
If you want to update a 3.11 Cluster, first install Padmin v.3.11.2 on the original nodes (file upload will be enabled on that version).
In that Padmin v3.11.2, set the RemoteLogin ports from 22 to 22022 (Padmin / Security settings / Supplier Login).
Let's change the port numbers node by node. (you may find error feedback, this just means the value could not be synced on each node automatically). Check all 3 nodes finally.
B. Switch off the first 'A' node of your existing Cluster. Make a backup (snapshot) of it.
C. Make sure that each of the 3 new VMs will be assigned to the same IP address range (subnet) as the original Cluster.
D1. You can also start them from V.center, where you can use changeip / changeip login to set the proper IP address of the VM to be replaced.
(the procedure for changing IPs from the command line can be found HERE)
D2. It's might be easier to get an IP automatically by DHCP (while switching on). You can correct ('fine-tuning' the IP address in the Padmin initial menu.
Make sure the first 'VM gets the IP address of your switched off 'A' node in the existing cluster.
E. (optional). Update Padmin with the latest file here in the INIT menu (only if there is a new padmin update-file released)
F. In the 'mode' menu, leave 'new cluster' empty (since this will not be a new cluster, but will be connected to an existing one). Enter the original Cluster IP address - you want to join to.
The port should be specified as: 22022 !
After a successful login, we need one more action: Maintenance: Reboot Local Server !
G. Do the same steps on the other 2 nodes. Continue with node B: Switch off the existing node, add join the new VMs to your cluster.
Finally, we get a stable, gradually updated Cluster.
PassMan and BaseOS upgrade can still be done via simple file-upload, which will be synchronized to all 3 nodes.