Hello @dg2k
Thank you for reaching out. I would like to confirm you that to change or replace Account Administrator on Azure Subscription you would need to perform Billing Ownership transfer for that specific subscription. More information about this is available on following document: Transfer billing ownership of an MOSP Azure subscription to another account. If you face issues while performing this or need more information on the process than you can raise a support request from Azure Portal with Azure Subscription Management Support Team.
I hope this answer helps to resolve your issue.
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