I agree with @nclark's advice above.
I made a few tweaks to the process below
- upgrade both hubs to the latest version of Hubitat firmware
- do the cloud migration save on my C7
- Disable the Zigbee and Z-Wave radios on the C7
- disable all cloud integration apps, and disable the parent HubDuino Device
- Perform an orderly shutdown on the C7 and then unplug it
- get new C8 running - register etc etc
- change router to associate old C7 IP address with C8 MAC address via the router's DHCP server
- restart router
- restart C8, and verify it is using the C7's old IP address
- Restore C8 from cloud to complete the migration
Here are some additional steps/tips from my migration from C7 to C8
