Apple watch on my Iphone

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 new phone 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.

First you unpair the watch from your old phone;

Unpair and erase your Apple Watch

Then you setup and pair the watch with your new phone using the backup you made when you unpaired from your old phone;

How to pair your Apple Watch with a new iPhone