Yes, if you do not have a system like a DS Lite to use, it is never safe to update your 3DS to a firmware that will block your card. The card can only be updated if you can run it without any blocks by Nintendo. Perhaps you can borrow a friend's DS Lite to update your card when a patch is released?
Well, damn. I don't know anyone with a DS Lite really. I do have my old DS, though (the first version)... Will it work if I do the trick there?