If the VM(s) are deployed using the Resource Manager (ARM) deployment model and we need to change to a size which requires different hardware then we can resize VMs by first stopping the VM.
In our case, we are not able to resize a live VM to B-series VM, we stopped(deallocated) VM from azure portal, then afterwords, we are able to resize it now.
Refer to Resize virtual machines
Finally, if need to get details about why could not resize VM, run below powershell script to print out errors
$ResourceGroupName = "YOUR_RESOURCE_GROUP" $VMName = "YOUR_VM" $NewVMSize = "Standard_B2s" $vm = Get-AzureRmVM -ResourceGroupName $ResourceGroupName -Name $VMName $vm.HardwareProfile.vmSize = $NewVMSize Update-AzureRmVM -ResourceGroupName $ResourceGroupName -VM $vm