Quoted from KSP1138:
It doesn't really matter if you are connected to the internet or not. If you swap SD cards that contain non-IC modified code and newer IC enabled code you shouldn't need to worry at all imo. There would need to be a mechanism in the hardware by which the IC enabled code could identify that a non-IC enabled modified SD card was used previously. And if they could do that, the IC enabled code could brick your machine, network or no network.
If they really want to stop modified code it would be easier for them to encrypt the SD card image and make it so the hardware would only run the encrypted images once you boot up with a newer version, if their current platform even supports that. I can't think of any reason they would go through the trouble because there's no financial incentive. Maybe if the licensors are giving them grief about it?
DirecTV could brick/loop hacked cards, and as the article mentions only went after the cards not the hardware itself. I'm told people would swap between hacked and non-hacked cards without detection, since nothing was flagged on the boxes. I'm guessing their estimates for the number of cards they took out with their big attack was based on the number of new subscribers right after that since they had no way of knowing otherwise.
I was going to quote and comment on each of these, but I just ended up at...I totally agree with you