I think it's totally gone, sorry.
I upgraded from an iPhone 7 to an X last weekend and let the Genius guide me through the change, which turned out to be suboptimal. I lost my health data - but got it back - but (again) I still had my original phone. Here's what I did:
When I got home, I took the VZW sim out of the X and put it back in the 7 and put the 7 back on the VZW network. I then wiped the Watch and re-paired it with the 7. I then did an encrypted backup of the 7 to my Mac in iTunes.
With that all done, I un-paired the watch, wiped the X, set up the X with a restore from the encrypted backup I'd just made for the 7, and paired the Watch with the X. Bingo, everything restored.
The two points here are (1) that because I still had the old phone and it was the same day, no data was lost (even though I had to essentially make the 7 my phone again briefly) and (2) I had an encrypted backup to restore to the new phone.
If you don't still have your old phone, you're probably out of luck. It's possible that if you've got an encrypted backup on a local machine or on iCloud, you might be able to restore it, but FWIW my encrypted backup on iCloud did not restore my Health information/history.