No, the swans would not scare the Great Blue Heron from the pond. If you were to get a pair of swans and they produced cygnets (baby swans), the Great Blue Heron would also predate the young birds.

The only viable way of protecting your koi is to enclose the pond (if possible) from predators such as egrets, herons, raccoons, etc. There is no guarantee that these predators would be prevented from accessing the koi by just scaring them. They would just wait until you leave and then come back in for a meal.

Additionally, Great Blue Herons and other such waterfowl are protected under most state and federal laws, so killing or harassing them may actually get you in trouble legally. The Regal Swan