code: func (c *Client) WalletPassphraseChangeAsync(old, new string) FutureWalletPassphraseChangeResult {
  	cmd := btcjson.NewWalletPassphraseChangeCmd(old, new)
  	return c.sendCmd(cmd)
  }
docs: // WalletPassphraseChangeAsync returns an instance of a type that can be used to
  // get the result of the RPC at some future time by invoking the Receive
  // function on the returned instance.
  //
  // See WalletPassphraseChange for the blocking version and more details.