Remember that when the attack starts, Gorkon tells them to find Chang. He's not with Gorkon at the time. I take that to mean that Chang was actually on the Bird of Prey planning, as others have suggested, to see Gorkon's ship destroyed. But when things went awry, he beamed back over to the flagship and just made it look like he had been there all along.Not arguing, but I always assumed that Chang was on the Bird o Prey all along and only made it seem like he was on the big ship. Chang doesn't strike me as the sacrificing type. But maybe that would have revealed it? He may have been ready to beam over to the Bird if the attack started. I still think it would have been a real Remember the Maine type incident for the Empire.
Also remember that his aid didn't get very far, running into the Starfleet crewmen with phaser first. Chang was likely on the bridge of Kronos One. Gorkon was elsewhere on the ship.