When I got my iPad and then MacBook Air I was still an android and windows user, so I used a gmail address for my Apple account. Eventually I used a custom domain and switched the account email address.
I am currently trying to wipe my MacBook to give to my daughter for college. But it needs me to sign out of find my and it has the original gmail address. It is not accepting the password, and when I do to find my nothing is signed in. Any tips? The only thing a search turns up is how to sign out nothing about it being a defunct account name.
If you changed the original account name it should still be the same account as the one you are using now. Turn off the mac. use another system to log into iCloud.com with your favorite browser, next go to find my. The device should be listed there. Click on remove from account. You can now wipe it and reinstall the OS with no issues.
I have it erase from my iPhone and it seems to be working.