Thanks for posting your question in the Microsoft Q&A forum.
If you absolutely need to change the original administrator username, you'll need to redeploy the VM.
While this method doesn't change the original username, it allows you to set up a new administrator account:
Set-AzVMAccessExtension -ResourceGroupName "myResourceGroup" -VMName "myVM" -Name "myVMAccess" -Location "WestUS" -UserName "newAdminUsername" -Password "newPassword" -typeHandlerVersion "2.0"
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