Thanks for posting your question in the Microsoft Q&A forum.
Here are the commands you might use:
az vmss update --name <vmss-name> --resource-group <resource-group-name> --set virtualMachineProfile.storageProfile.imageReference.id=<image-id>
Ensure that the account used to authenticate has sufficient permissions in both subscriptions to read and update resources.
Keep in mind that updating the VMSS image will trigger a rolling upgrade of the VM instances in the scale set, during which each instance will be updated with the new image. This may result in downtime for your application, depending on your configuration and update strategy. Therefore, it's essential to plan and execute this operation carefully, especially in production environments.
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