I did read your first reply in detail. I understand it. But I'm not making myself clear and frustrating you in the process, so here's another attempt.
Our development network isn't internet-connected, so installing an extra tool is a non-starter. For the time I'd have to take to set up new repositories to host its code on our network and fake out the installer through DNS trickery, it's not worth the headache, especially when the Zimbra-approved method is documented as at
https://wiki.zimbra.com/wiki/How_to_mov ... her_server
I have two servers:
prod.my.net: Currently in operations. Zimbra 8.0, CentOS 6.9.
new.my.net: Not in ops. Zimbra 8.8.9, CentOS 7.5. Data rsync'd from prod.my.net but that data has not been referenced yet, as the services haven't started on this system yet.
Can I downgrade prod.my.net from 8.8.9 (which has never been set up or executed) to 8.0.6? I imagine just removing all the RPMs and reinstalling using the install.sh from 8.0 ought to do the trick (basically, what you suggested in your second reply above). If not, then we just need to wipe the VM and rebuild it before installing 8.0 on a clean OS. Then, re-rsync the data and perform the upgrade.
And yes, I do know that I'll need to take prod offline to do the rsync properly, and not have messages being written to prod that won't ever be part of new. And I'm sure the Zextras packages would take care of that, but again, mine isn't a typical installation.