alterar icone do item de um menu antes de inflar

@Override
	public boolean onOptionsItemSelected(MenuItem item) {
		Log.i("erro", "" + item);
		//super.onOptionsItemSelected(item);
		
		 switch (item.getItemId()) {		
		 		        
		 case android.R.id.home:
			 menu.toggle();
			 
			return true;
			// break;
	        case R.id.mapa_:
	        	Toast.makeText(getBaseContext(), "Clicou no mapa", Toast.LENGTH_LONG).show();
	        	 mapType = GoogleMap.MAP_TYPE_NORMAL;	        	 
	        	 
	        	//item.setIcon(R.drawable.ic_location_web_site);
	        	
	           return true;
	        case R.id.satelite_:
	        	Toast.makeText(getBaseContext(), "Clicou no satelite", Toast.LENGTH_LONG).show();
	        	mapType = GoogleMap.MAP_TYPE_SATELLITE;
	        	//item.setIcon(R.id.mapa);
	        	//menu.getResources().getXml(R.id.mapa);
	        	
	           return true;
	        }
		myMap.setMapType(mapType);
		return super.onOptionsItemSelected(item);
	}

public boolean onCreateOptionsMenu(com.actionbarsherlock.view.Menu menu){
		MenuInflater inflater = getSupportMenuInflater();
		inflater.inflate(R.menu.activity_main, menu);
			
               // estou com problema aqui na lógica...

		menu.findItem(R.id.mapa).setIcon(R.drawable.ic_location_web_site);	
		
		return true;		
		
	}