Marlin,

Shal, if the member gives permission for his message to be transferred
and made available on an entirely new system, how can you cope with one
of his messages being a "reply" that contains the original message
written by yet another member who may not like the idea at all?

I don't know a good answer to that.

Keep in mind that the TOS and Copyright aren't about privacy, and Copyright law does allow copying a portion of a copyrighted work in some circumstances as "fair use". That said, and bearing in mind that I'm still not a lawyer, I don't know if fair use would extend to a complete copy of the message (as when most people top-post). That's one of the reasons I prefer to inline post, using only snippets of the original message.

My guess is that the offended member would get nowhere with their complaint under the operation of US copyright law. If the complaint were of an invasion of privacy that would be highly dependent on the circumstances, such as whether both groups had public or private archives, or were different, or if there was a substantial (initial) difference in the membership of the two. Probably still would get nowhere given that the offended member originally posted the information (nobody "invaded" their privacy), but a good lawyer can argue any point of view.

Actually, I do know a good answer to that: automatically strip leading or trailing whole quotes as part of the normal display of messages on site.
https://groups.io/org/groupsio/beta/message/1403
https://groups.io/org/groupsio/beta/message/1433

-- Shal