The answer is : depends on the type of the game.
For example, if the game is a racing 3D game, you can modify the aspect ratio of the screen, but all the assets stay the same. If it's a board game with zoom - the same applies - you will just see more of the board.
The main problem will be, IMHO, GUI. Note however, that in some type of games it would be perfectly ok to leave it exactly as-is; it would make steering with on-screen controls easier, while keeping the gui visible.
As the footnote: iPhone 5 has the same horizontal resulotion as 4S. Whilst you can just play 4S games on 5 in letterbox mode, opposite isn't possible. The letterbox space can be filled not only with extending of the game display, but, as I mentioned earlier, larger gui or controls