Oh well. Once again a what you would think common product and scenario is simply undocumented.
So here it goes: "How do I migrate a NCS Clustered Messenger 3.X system to Messenger 18.1?"
Apart from the documentation mentioning clustering like it is supported, but then *completely* failing to actually document it (I'll get to that later), it seems that in the installer clustering and migration from 3.X are mutually exclusive. At least, when you specify migrating a 3.X system in the configuration, no option for clustering is shown anywhere, and if you try to "manually" do it by specifying the cluster resource IP and shared storage everywhere, the install fails like this:
Error initializing database connection: 0xB407
Failed. Error connecting or authenticating to the database.
1. User root does not exist.
2. Password for root was entered incorrectly.
3. Database server address [X.X.X.X] and/or port  are incorrect.
4. SSL is not configured on the database server.
Which only adds to the confusion, especially the mentioning of the root user, as it never ever asked for the root password in the first place.
The reason for above seems to be a rights issue on the shared storage, but again, that's not my job to find and debug, as it's not documented.
Also, the arango database itself gets created locally when you specify migration, which is totally not what is wanted in a cluster scenario.
Now, for the documentation:
we find this sentence on Clustering:
"The Messenger agents can be configured to take advantage of the fault-tolerant environment provided by Novell Cluster Services. The installation program adds a /cluster switch to the Messenger agent startup files. This tells the Messenger agents to use the cluster virtual server name."
As for *HOW* the Messenger agents can be configured that way, there is *zero* information anywhere. I so love this type of information that simply leaves you stumped.
However there is also a link "For more information on clustering Messenger, see the GroupWise 18 Interoperability Guide." Maybe that helps, but...
Which again contains *zero* information on clustering Messenger.