Hello VonPhillipd,
Thanks for your post in this community.
We read your entire post and updates here. As there is no other person has admin rights in your organization, so in such situation, first you need to reset that past admin account then need to change payment method by follow the steps mention in this article. For reset admin account, need special resources and permission and as we are belonging from Microsoft365 community site team, have limited resources for the same. So it is recommended to kindly reach our technical team via Phone support, (Billing team also available via phone support) they will assist you further. Your co-operation will be highly appreciated.
One more thing, admin account password reset if he has set up alternate email address and phone number. So if needed with the help of past admin’s family member, you can try to check if you can reset such past admin’s account password. During password reset if its promoted for such alternate email id and phone number, ask his family member to check if they have some details for the same and reset it. For reset password, you can use this link https://passwordreset.microsoftonline.com. For your reference, here sharing article for An administrator forgot his or her password in Office 365.
Thanks for your precious time. Stay safe and healthy.
Regards,
Ankita Vaidya