I want to connect my apple watch to my new iphone but I don’t remember my old apple id

You're going to have to figure out the old ID before you can pair the watch to your new iPhone.

This link may help you recover the ID --> If you forgot your Apple ID email address or phone number - Apple Support

If you simply can't recover the Apple ID you used on the watch, your ONLY hope is you have the original purchase receipt, which clearly shows the watches serial number on the watch. With that and a photo ID, Apple will help you. Without it, Apple won't.

This is a classic example of why it is NEVER a good idea to change your Apple ID. You can change the email address associated with an Apple ID. You can change the Apple ID Password for your Apple ID. But creating a new Apple ID is never a good idea.