I don't understand the idea of the Oathbreaker mechanically. Paladin is literally the only class that has a built in self-destruct consequence for role playing decisions.I like the Eberron/Keith Baker take that so-called divine power is not necessarily tied to the gods but rather to some internal force of will. Or if it is an external divine gift, then the gods are too big to be concerned with one individual misusing their divine gift, or the gods have machinations that are not so easily understood by mortals and our limited grip of morality. Thus evil clerics working in service of good gods without losing their powers.
I don't see this as much different for paladins. Their power may be focused by their oath, but the oath is the tool and not the source. A Fighter doesn't stop being a Fighter just because they lose their sword, nor does the wizard stop being a wizard if they lose their spell book. I can see things being harder, but they don't necessarily suddenly become something else.
I feel like if the player is making decisions and doing things that are more relevant to RP, then the consequences should be similar. Such a paladin might be outcast from his order, hunted by his former friends, forced to live with the memory of innocent blood on their hands, or so much more. All are more interesting and less handicapping mechanically than changing the class or how it functions.
But in answer to the OP, if you did become an oath breaker, I would allow the smite to deal necrotic. It's balanced against radiant and is conceptually aligned with the character.
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