What if I don’t remember my old Apple ID that was used to set up my watch and I have a new phone to pair it to?

See: If you forgot your Apple ID email address or phone number - Apple Support

Or if you just forgot the password: If you forgot your Apple ID password - Apple Support

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

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

Remove the Activation Lock 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.