Change disk driver VirtIO to Virtio-SCSI

Hi,

We have an Openstack environment with about 200 instances.
These write a lot to Ceph, and we figured out that running `fstrim /<mountpoint>` can clean up a lot of raw ceph disk space.
Unfortunately all of our instances use the VirtIO driver, but we need Virtio-SCSI.

Is it possible to change this for existing VM's, without recreating them from snapshot or rebuilding them?

I tried adding the properties `--property hw_disk_bus=scsi` and `--property hw_scsi_model=virtio-scsi` to the flavor, image and instance, but this didn't change the current driver.

EDIT: WE are currently running Stein release (yes it is very old, we are soon migrating to 'the cloud' unfortunately).
EDIT2: We need to have the VirtIO-scsi driver on a cinder volume, if that changes anything.

Please advice, thank you!!