Yes, my assumption is that Sarnia keeps aa card open for Konecny and Chychrun for example. My assumption also was that the players all have cards assigned to them regardless, not obtain one only when they are sent back to the OHL from the NHL.
I always assumed that if a team kept the player on card regardless of whether the player is in the NHL or not, that the player could return whenever. Cards are limited of course and there is no incentive to do this as NHL teams simply do not decide to send players down with 10 games left in the NHL season.