In the manual it should be clearly stated the role of tenant admin and that it cannot be changed once tenant has been made. Maybe even have suggestions of using names like "tenant-admin" or "tenant-purpose-x-admin" as the name. Similarly mention should be made that tenant admin should never be disabled.
UPN and ID are embedded in the Tenant structure.
Apart from the UPN and the ID, you can change any other content you want (name, email, etc.) so you can have someone act as tenant admin without creating them as admin-person to which tenant then would be tied to.