As I understand it, the software itself never really gets changed, because if it did then the whole plant might have to undergo testing and regulatory approval

As I understand it, the software itself never really gets changed, because if it did then the whole plant might have to undergo testing and regulatory approval again. But the reactors get shut down periodically (every three years, maybe) for refueling, so the control hardware undergoes routine maintenance at that time. Or something like that.