If they were able to cause the pacemaker to fire when they wanted they could time it during the repolarization, which could possibly cause a fatal arrhythmia even in a heart that doesn't need a pacemaker. It's called R-on-T phenomenon and it's usually caused by malfunctioning pacemakers.
I doubt you could do that through configuration changes alone, simply because of how defensive the firmware would be about that exact scenario. You'd probably have to resort to code exploits on top of simply gaining access. Even then, there's probably a rudimentary interlock at the silicon level.
