Pairing my old watch with new phone. I do not have pw from when I set up this watch.

If you forgot your Apple ID password

If you forgot your Apple ID

Setup your Watch to a new iPhone

You can ask apple to remove the activation lock online on this page;

Online Activation Lock Removable Here.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt with the serial number along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.

You don’t. You will need to recover access to the account you used when you set it up.