IBM (Lotus) has made no commitment to support DUC on the AS/400 platform. So it's not going to happen any time soon, anyway. As usual, if you pass along specific opportunities to the Unity Product Management team, that will help them form a business case.
About VM to UM: I can't answer the license issue definitively, since it's out of my area, but I would expect so. The actual migration is going to be a complete re-install, however, so keep that in mind.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.