Across both the core Iron Man films as well as others in the MCU, Tony has designed a number of suits to suit various mission needs and protocols.
As such, the triangular design of the Mark VI armor you're asking about could have simply been an aesthetic choice for this particular suit, and not because the power core in his chest changed.
Note that the Mark VII armor, which comes after the model you're talking about, sees a return to the circular unibeam shape.
However, as shown when Tony activated the "House Party Protocol" in Iron Man 3, and early in Age of Ultron when we see a facility in the Avenger's tower manufacturing robots and suits, Tony has become quite paranoid since his battle in the first Avengers movie (which took place between Iron Mans 2 and 3), feeling it's up to him to protect the planet the only way he knows how: with a suit of armor, or in this case several suits of armor around the planet.
Tony is constantly upgrading and modifying his suit, as any gearhead is want to do. Given the Mark V (aka, the Briefcase Suit) was damaged early in Iron Man 2, and it's possible there was still damage to the Mark IV due to his drunken antics at his party, where he got into a fight with Rhodey who was wearing the Mark II suit, the only one available at the time was the Mark VI, which simply happened to have a triangular unibeam chest plate.
Other suits have had different unibeam centers on their chest plates, an extensive number of which can be seen here.
Most are circular, but some have vertical and horizontal rectangles, others are triangular and even have triangles with flattened corners, and even the size of the unibeam emitter changes depending on the suit and its intended purpose.