How do you pair the watch on a new phone? Take it off the old one.

You MUST unpair the watch from the old iPhone before you can use with a new iPhone. For complete instructions (so you don't lose any data), see:

How to pair your Apple Watch with a new iPhone - Apple Support

To unpair and erase watch and remove Activation Lock

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.

Then;

How to pair with my new phone.