Thx guys. For some reason, the "SageTVPluginsDev.xml" rename did not work for me. But if I instead move the original SageTVPlugin.xml out of the way, and move Repo.xml in place (only renamed as SageTVPlugin.xml), then upgrade, all is well! Back to a fully functioning Gemstone interface.
Thx all for your help!
I've now made that SageTV directory backup I've been meaning to do for weeks now :-)