When you say you changed the Apple ID, did you do this by truly changing it, or did you do this by creating a new account? To me it sounds like the latter. You will need access to the old account.
Click on this link to read what to do "If you forgot your Apple ID [Account] password" --> https://support.apple.com/HT201487
or go directly to this link --> http://iforgot.apple.com/
Also read about "What to do after [or before] you change your Apple ID email address or password" --> https://support.apple.com/HT205009
Click on this link to read "About Activation Lock on your Apple Watch" --> hhttps://support.apple.com/HT205009