Are we talking about you having changed the Apple ID, or about you having created a new Apple account?
Click on this link to read what to do "If you forgot your Apple ID" --> https://support.apple.com/HT201354 Note that typically this is an email address you use.
Click on this link to read what to do "If you forgot your Apple ID password" --> https://support.apple.com/HT201487
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).