Change storageAccountType of VM Scale Sets Issue

Hello,

I'm trying to convert from Standard_LRS to Premium_LRS to a vm scale sets but it gives out an error "changing property of "osDisk.managedDisk.storageAccountType is now allowed" .

I'm using a custom image, which the image itself was converted to Premium_SSD through PowerShell.

Any advice is greatly appreciated. 

Thank you.