Fundamentally there isn't any change in the underlying physics. You're limited by the half life of the isotope, so you either choose a longer-lived isotope (whi

Fundamentally there isn't any change in the underlying physics. You're limited by the half life of the isotope, so you either choose a longer-lived isotope (which actually may be even harder post Cold War as much of the ideal isotopes were byproducts of nuclear weapons manufacturing) or launch a bigger RTG than you need so you have enough power later on in the mission.