Ok here is the way it has to be.
Keep his H3 Armor, especially in the beginning. Don't try to be fancy because it just ruins the experience.
Since we don't know the story it would be hard to explain why his armor would change. Quite frankly I think it is funny how he gets new armor every game just to please the fans. If he does upgrade let it stay for a game or two.
One thing that has always bothered me is that we never got to see, even if through a cutscene from Guilty Spark the level 12 armor. They could go all Clash of the Titans and just outfit him for his final battle haha.
His look in the Halo 4 Tralier is almost leaning toward a sleek Forerunner look, although not too much. He reminds me of the main character from Tiemshift in some way.
If he gets new armor in the game then so be it, but I am sure we won't see him change his helmet haha.