I'll attempt to describe this strange issue. I've been using HE for a few years (C5 hub). I bought the C8 hub during the holiday sale and performed a migration. I've decided to return the C8 and continue using my old C5. It was working well for me anyway and I only upgraded because it was on sale, so why spend the extra $$. 
When I unplugged the C8, I realized that all my Zigbee devices stopped working on the C5 hub. Everything else was working fine on both hubs simultaneously (incl. Sonos, Lutron pico remotes, and Philips Hue bulbs). It's jus the Zigbee devices which don't work on the C5 anymore.
So, I tried resetting some of the devices and pairing them back to the C5 hub, but it doesn't seem to be working. For example, for my ST buttons, I removed the devices from both hubs, reset the buttons via instructions and tried to pair them back into the C5. The C5 shows that it's been detected, but it never gets added and just times out.
Is this something new? I wasn't having this issue before the migration. If anyone has thoughts, or wants more information, it would be much appreciated.
Is the C-8 still online? It could cause problems if so. Otherwise, resetting and re-pairing the Zigbee device should work (no need to remove it from the hub first, and it's more work that way since you'll have to put it back in any apps you were using -- not so lucky if this were Z-Wave). Most LAN integrations, including Sonos, Lutron, and Hue, are perfectly fine working on multiple hubs at the same time. But having to re-pair most Zigbee devices is, as documented, expected if you try to "un-migrate."
Since you'll have to re-pair everything anyway, you could also reset the Zigbee radio to see if that helps. If you're on 2.3.7, the easiest way to do that at the moment is to downgrade to 2.3.6 (using the Diagnostic Tool) given some recent changes, then re-upgrade.
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Wow. That did it! When I powered off the C8 hub, the pairing worked perfectly on the C5. I wouldn't have thought that was an issue as only the C5 was actually doing the pairing process, but who am I to question the voodoo magic. 
Yes, this is why the migration instructions tell you to shut down the old hub and remove power. After migration, the Zigbee radios will share an IEEE address, and without a reset, both will also share the same PAN ID and (unless you changed it) likely even Zigbee channel.
