'Convert the volume to be of the new type.
:param ctxt: Context
:param volume: A dictionary describing the volume to migrate
:param new_type: A dictionary describing the volume type to convert to
:param diff: A dictionary with the difference between the two types
:param host: A dictionary describing the host to migrate to, where
host[\'host\'] is its name, and host[\'capabilities\'] is a
dictionary of its reported capabilities.'

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

    LOG.debug('retype    volume    id=%(volume_id)s    new_type    id=%(type_id)s', {'volume_id': volume['id'], 'type_id': new_type['id']})
    return self.common.retype(ctxt, volume, new_type, diff, host)
