https://wccftech.com/psa-watchos-8-rc-is-here-downgrade-to-watchos-7-is-not-possible/

Apple has seeded watchOS 8 RC to registered developers and public beta testers. You can't downgrade back to watchOS 7 once you make the jump.

It is not a secret at all that Apple does not allow downgrading back to older firmware on Apple Watch. Although we've mentioned this clearly several times in the past, but it is still worth a shout at this point as well since watchOS 8 RC is out and many people will be updating to the firmware in sheer hype.