If your familiar with Cocos2d, it will be pretty simple.
We use sprite sheets for the art, so that will help.
But if you really wanted to you could re-use some of the art assets and change just the major art assets that would make your game unique to get approved in the app store.
The coding is all done, just doing some switching of the art would all you need to do. But it's a plus if you have some knowledge of Cocos2d just so you can review the documentation and modify files/art if needed.
Hope that helps.... also check out the instructions I posted to reskin the game. It is at the bottom of the Game Description.
Addicting Springboard Multiplayer Game (iOS8 + Cocos2d)