'Convert the volume to be of the new type.
Changes the volume\'s QoS policy if needed.'

def retype(self, ctxt, volume, new_type, diff, host):

    qos = self._get_qos_by_volume_type(ctxt, new_type['id'])
    if (not qos):
        qos = COHO_NO_QOS
    self._do_set_qos_policy(volume, qos)
    return (True, None)
