Hello Alan,
Please check
https://www.ovirt.org/documentation/how-to/hosted-engine/#migrate-hosts-from-el6-to-el7 to understand how to perform your upgrade from EL6 & RHEV3.5, quoting:
"In 3.6, el6 is not supported anymore for hosted-engine hosts.
Existing 3.5 el6 hosts should be first migrated to el7, then upgraded to
3.6. More details in
Hosted Engine host operating system upgrade Howto."
It is up to the Engine (RHEVM) to be aware of the networks on your
hosts (and to configure them through running VDSM daemons), so if you want the tagged networks the way you configured them
manually on your hosts, you need to define the cluster networks in web
GUI correspondingly.
I don't think it is possible to leave network setup to manual configuration by standard means (that is, by having VDSM running to take care of all but networking chores, but have it patched or use hooks to implement your configuration).
I, too, don't exactly understand your existing setup. Can you describe it in more detail? Since you want to learn how to perform an upgrade, you should try to mimic your production system, which is necessarily managed by VDSM daemons running on all hosts.
If you are having a trouble reproducing your production environment in your lab, please describe the steps you took. Perhaps it is reasonable to install the Engine first with no complicated network setup, and only then configure networks step by step.
Thank you,
Ondra