Android :: Alter MenuItem On The Options Menu?

I have an Options Menu on my Activity with an MenuItem "Start". When this MenuItem is selected I would like to alter the Menu so it contains a MenuItem "Stop". And finally when "Stop" is selected, I would like to alter back to "Start". Here is parts of my code that isn't working. I have both "Start" and "Stop" listed in mymenu.xml I should probably remove "stop" when the menu is created:

public class MyActivity extends Activity {
private boolean isStarted = false;

@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
menu.removeItem(R.id.stop);
inflater.inflate(R.menu.mymenu, menu);
return true;....................