What I meant to say was, create a new harddisk with the new controller type.
Like this
$vmName="TestVM"
$VM=Get-VM-Name$vmName
New-HardDisk-VM$vm-CapacityGB1-ThinProvisioned|New-ScsiController-TypeVirtualLsiLogicSAS
Then reboot the VM, and the guest OS should take care of the driver.
Once the driver is installed in the guest OS, you can move the harddisk containing the system partition to the new controller.
$vmName="TestVM"
$VM=Get-VM-Name$vmName
$controller=Get-ScsiController-Name"SCSI Controller 1"-VM$vmy
Get-HardDisk-VM$VM-Name"Hard disk 1"|Set-HardDisk-Controller$controller-Confirm:$false