An implantable cardioverter/defibrillator device is implemented to be selectively non-invasively upgraded from time to time after implantation to enable the device to provide additional therapy for arrhythmia treatment as the patient's need for such treatment undergoes change. The device is adapted to provide a plurality of functions corresponding to different levels of therapy for treating arrhythmias, and to respond to each different type of arrhythmia that may be sensed, to supply a function which is designated as being appropriate to relieve that respective arrhythmia. Each function is not necessarily unique to treating a particular arrhythmia, and, in at least some instances, may be used to treat more than one of the plurality of different types of arrhythmias. At the time of its implant, the device is restricted from providing those of the plurality of functions which are deemed as being non-essential to the patient's needs at that time. From time to time thereafter, however, as the patient experiences periodic need for additional therapy, restricted functions of the device are selectively restored by external programming, but only if the programmer is able to supply to the device an enabling code which is substantially unique to that device. In this way, restoration of the restricted functions is locked out except with a prescribed key.