code: func (n *Node) DowngradeKey() error {
  	paths := ca.NewConfigPaths(filepath.Join(n.config.StateDir, certDirectory))
  	krw := ca.NewKeyReadWriter(paths.Node, n.config.UnlockKey, nil)
  
  	return krw.DowngradeKey()
  }
docs: // DowngradeKey reverts the node key to older format so that it can
  // run on older version of swarmkit