I've had no problem re-booking VIP GO tickets. It often requires the agent a bit of time to figure out how to handle the VIP fares, but when they finally do, I've had no problem in terms of changing dates. In one case, the agent converted the VIP GO ticket to a normal SAS GO ticket. But in all other cases (perhaps 5-6 in the last two years), they tickets were rebooked and still VIP.
But maybe things have changed - last change was in Dec 2018. Or it could be a case of keep calling until you find an agent who can help!