Pacemaker firmware can almost always be updated using inductive or rf telemetry. In most cases it still requires an appointment with a cardiologist or similar physician though.
And many can now be monitored by the cardiologist from their office as the device uploads data to a server, or can even be reached directly from the physician's console. And both cardiologists and pacemaker companies generally have a pretty good bead on who's walking around with which serial numbered device.
