How to replace a VM's disk with a new one with Az cli

In this scenario we will replace the os disk of a VM with a new one moving from HDD to a SSD. Here are the steps:

Set your variables

$rg = "ResourceGroupName"
$vmName = "VM"
$oldDiskName = "vm-os-disk"
$newDiskName = "vm-os-disk-premium"

Stop the machine

az vm stop \
  --resource-group $rg \
  --name $vmName

Create a snapshot of existing disk, specify the new disk SKU (we move from Standard_LRS to Premium_LRS)

az snapshot create \
  --resource-group $rg \
  --name $($vmName)-snap \
  --source $oldDiskName \
  --sku Premium_LRS

Create the new disk from the snapshot

az disk create \
  --resource-group $rg \
  --name $newDiskName \
  --source $($vmName)-snap \
  --sku Premium_LRS

Attach new disk to VM

az vm update \
  --resource-group $rg \
  --name $vmName \
  --os-disk $newDiskName

Start the VM

az vm start \
  --resource-group $rg \
  --name $vmName