If you could still login to the Registrars user account that owned the domain and that email address had been changed I suspect you should still be able to chan

If you could still login to the Registrars user account that owned the domain and that email address had been changed I suspect you should still be able to change the domain contact information via the self service console. Worst case you'd raise a ticket (how good is actual support!) and they would be able to change the domain contact information for you because you can already authenticate with the user account that owns the domain. If this wasn't possible all those domains that have self referencing contact information (eg. abuse@this-same-domain.example) could never be verified.