code: public void changePassphrase(String oldpassphrase, String newpassphrase)
  			throws IOException, InvalidPassphraseException {
  		SshKeyPair pair = toKeyPair(oldpassphrase);
  		formattedkey = encryptKey(pair, newpassphrase);
  	}
docs: /*
  (non-Javadoc)
  
  @see
  com.sshtools.publickey.SshPrivateKeyFile#changePassphrase(java.lang.String
  , java.lang.String)