The Cubase doc that came with the Zoom processor makes no mention of version and certainly not which to download. After speaking with support, that appears to be by design. Martin, you’re correct that the download keys in boxes are statically linked to a particular version. But after a new version comes out, in their license server, they identify those unused keys still in boxes as being able to download/activate the latest. They haven’t done that step yet for V10. He emailed me a V10 key; all is well. I’ll bet they’re getting a lot of calls like mine… 
NB: Note for newbies like me: Top-level “Devices” menu is now “Studio” which made following the as-yet not updated youtube tutorials sort of difficult.