I completed a migration from C-7 to C-8. Everything seems to have worked just great.
My question is in regards to the C-7. What the best way to reset everything on it? If I power it up and connect, will it mess with the Zigbee/Zwave pairings on the C-8?
If you leave it running too long, yes. I think the Zigbee PAN ID will actually always be the same as the C-8 now so you can only use the zigbee on different channel. A Zwave reset will change the zwave network ID.
First, you may want to wait a week or so in case you need to emergency revert back to the C7 for some reason.
Once you are happy with the C8, fire up the old hub, and then go to zigbee and zwave details pages, and reset both radios. After that is done, use the diagnostic tool to do a soft reset. Then you have a fully clean slate.
If you are going to sell or give away the hub log into the Hubitat portal site and de-register it from your account.