No your Watch will be fine.
I was on the iOS 14 Beta and had my watch on OS 6 for the entire time.
I’m now on the full release of iOS 14 and I’m waiting for watchOS 7 to be available later today.
Caveat: this is how it should work, but as with anything in technology, glitches can happen so make sure you do a back up first.
Encrypted backups include a backup of your atch as well.