The managed OS disk storage account type is a property that can only be set when the VMSS is created, please see the link for reference.
Which means you cannot use the "-ManagedDiskStorageAccountType" parameter for updating the VMSS with Update-AzVmss cmdlet once the scale set is deployed. Work on document update is in progress to make this clear and avoid confusion, please see this link for reference:
Thanks.
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