I presume it is a hardware revision, like how Tesla removed radar.
That's why Tesla gets cool features like door open pedestrian/car alert, child left behind detection, smooth stop etc over the life of the car.
It just needs to start a charging session, and then siphon energy _off_ instead of supplying it. People tried that before, but the firmware on Model 3/Y monitors the flow and terminates the session if the flow is negative.