@stackers Follow the instructions in this Support Guide:
https://answers.netlify.com/t/support-guide-another-site-is-using-this-domain-verifying-domain-ownership-for-netlify-production-domain/13765
The value you would use is:
https://answers.netlify.com/t/lost-access-to-old-account-cant-use-domain-on-new-one/128058