OK, I'm in the "new OS" and, as noted in the Support Ticket (94657), tried to install ver 8.3.5, got an Error: "Another version...is already installed. Installation cannot continue....Add/Remove...".
I fear that, if I were to remove ver 8.3.x, then try to reinstall ver 8.3.5 on this OS, it will also remove it from the "old OS', and, when/if I were to try to reinstall ver 8.3.5 on the "Old OS", I'll just get the same Error Message, catch-22.
I'm afraid this is not fun any more, unless we can find a way to "force install" ver 8.3.5 on the "new OS" without interfering with the one on the old one.
All ideas welcome (copied to the Support Ticket)