In real terms there is no downgrade. You would need to return your watch to Apple Service to be completely refreshed back to original factory condition, and you would be unable to update again - ever - because it would find WatchOS 10 again.
Apple's service agent was correct, the battery behaviour of your Apple Watch should revert to its previous life after a day or two. Almost everyone who does a major version upgrade sees reduced life for a few days while the new software cleans up and reindexes. That's a lot of extra hidden work which uses extra power but it is only temporary. Just be patient and don't try to stop it happening, because you can't.