Upgrading from C5 to C8. If I do the upgrade and it don't go well is it easy to revert back to the C5 until I figure things out? Don't want to be down to long. Do I need to do a cloud backup or can I do this from a local backup?
It is very easy, just unplug the C8 and restart the C5 and you're back where you were.
for good measure I'd say reboot the C5 with DB rebuild option and make sure the logs are clean, no errors and disable any LAN or cloud integrations before the cloud migration backup
Read carefully all the way through. How to Migrate to a New Hub | Hubitat Documentation
This ^^^^
You must use a cloud backup. If you don't have a Hub Protect subscription, you will create a Migration backup on the C5 and use that to restore to the C8.
If you do have Hub Protect on the C5, then you just create a normal Cloud Backup on the C5 and then use that to restore to the C8.
Please actually read the migration instructions linked to above - important details there.