code: func accountUpdate(ctx *cli.Context) error {
  	if len(ctx.Args()) == 0 {
  		utils.Fatalf("No accounts specified to update")
  	}
  	stack, _ := makeConfigNode(ctx)
  	ks := stack.AccountManager().Backends(keystore.KeyStoreType)[0].(*keystore.KeyStore)
  
  	for _, addr := range ctx.Args() {
  		account, oldPassword := unlockAccount(ks, addr, 0, nil)
  		newPassword := getPassPhrase("Please give a new password. Do not forget this password.", true, 0, nil)
  		if err := ks.Update(account, oldPassword, newPassword); err != nil {
  			utils.Fatalf("Could not update the account: %v", err)
  		}
  	}
  	return nil
  }
docs: // accountUpdate transitions an account from a previous format to the current
  // one, also providing the possibility to change the pass-phrase.