What data history will I lose when I erase my AppleWatch to repair it with my new phone?

If you unpair the watch it will make a backup.

Then when you setup the returned watch or another watch you can restore from that backup.

Most data is in the backup. However, note that Fitness/Activity data is NOT in the backup. It usually periodically during the day gets synced to the phone and if you setup another watch any synced Fitness/Activity data for that day will be synced to the "new" watch.

My experience is that fitness data like Stands are not in the backup since many times I have restored my watch from it immediate backup and Fitness data was not restored. I noted that only dat that was in the Activity app on the phone is re-synced to the restored watch.

I have had problem syncing Fitness data to my phone sometimes and have lost Fitness data for the day. If it was in the backup and showing on the watch before unpairing then it should go back to the watch but my experience is that it is not.

When you unpair your watch a backup of your health and fitness data is made automatically as long as the paired phone is signed into iCloud and the backup is uploaded to icloud,when you come to re-pair your watch you are asked if you want to set it it up as new or restore it from a backup,you choose restore from backup then all your health and fitness data is downloaded back to the phone and watch from the icloud backup and you carry on using your watch with all previous data intact.

I've never lost a single byte of data from backups or restores.