Hi, Dave. Thought I'd share that I just did an upgrade via DVD frm RH8 to RH9 and was also surprised that it didn't recognize that I had RH8 and do an actual upgrade. Adobe says it does a clean new install and confirmed that you can have both versions installed, although Forum MVPs recommend against doing that. I'll be removing RH8 once my projects have been updated.
Sorry that I can't help with the download part since my company went the DVD route.
The person with the RH9 license will have to first deactivate their copy. Then you can download the trial & use their serial number to activate. RH8 & RH9 can co-exist - just ask Rick (Captiv8r) - he's got versions going back to X5!
The main thing that doesn't put up with multiple copies is Acrobat - you can only have 1 of those (and according to Dov Isaacs' best practices, don't have both the Reader & the Distiller on the same machine).
Thanks very much for both answers.