What Are The Risks Of Not Changing Batteries Of A Defibrillator?

My dad has had a difribulator for 6 years now. The doctor is now saying they need to replace the batteries in it- which consists of the full removal of the device. This carries ALOT of risks, and he is 83. My question is- Is this absolutely necessary/? It has never given any signals in the 6 years... Can we just leave it in to avoid the life risks and not replace it or does it have to come out if the batteries are dead?