To replace iCal icon: 

1. Copy App-empty.icns and App.icns
2. Go to Applications, Right click on iCal->Show Package Content. Go to Contents/Resources. Rename files App-empty.icns and App.icns to App-empty_old.icns and App_old.icns for backup.
3. Paste new App-empty.icns and App.icns

Now icon will not change to default.

To restore default icon - delete App-empty.icns and App.icns files from Contents/Resources folder, and restore names of App-empty_old.icns and App_old.icns changing them to App-empty.icns and App.icns