Burke and Samno did. For all we know Cartwright or Valeris orchestrated it. There's no evidence that Chang was involved with that part of the plan.
Oh, yeah. Him being on the ship when it happened, not being anywhere around Gorkon during the massacre, and of course him being the captain of the Klingon Ship that fired torpedoes at Chronos One! Ya, it's easy to see how he had nothing whatso ever to do with the plan to assassinate Gorkon