Nerfs are indigenous to Alderaan, at least in Star Wars: The Old Republic.
Nobody ever thinks about all the innocent animal species that Tarkin extincted in A New Hope, do they?
The housing for the chest piece is circular, but the element that it houses (and what gives off the light) is triangular. I think they just made the piece in the armor to match thematically -- a new Tony with a new "heart", and a new suit to match his call to action in the third act.
Still not seeing it as "triangular inside the circle" but OK. It's not like I'm ever watching Iron Man 2 again. ETA: Hmm-- watching the new trailers for 3 and now I am seeing it. OKfine.
I prefer to suggest that the round Arc Reactors we see in Avengers are a further evolution of the 'fixed' design, refined back into a circular configuration. I can live with that.