Since you were rebooked instead of having to buy a new ticket, it sounds like it was not your fault. I would first try to retroclaim the originally booked flight on the A3 website (although you may have to wait until two weeks have passed before you can do that). If that fails, contact A3 customer support and ask for original routing credit (ORC), according to the booked flight.
Try your utmost to get this sorted before your SK flight in May, otherwise it will be a lot more difficult to resolve (if those flights would go towards A3*S instead of A3*G). I spent months trying to get a flight retroclaimed a year or two ago, so I wish you luck!