I mean if it's mechanically the same as a Paladin under the hood but you'd rather it not be called "paladin", we could call it Void Knight or something I guess. But if it's a paladin under the hood, has a beacon placing healing spec, a shield throwing tanking spec, and a two-handed melee dps spec, has the same exact skills as a paladin but with different names and visuals... is it really a completely different class? A completely different theme for sure, but mechanically speaking it would still be a paladin no?
As to the lore, the lore doesn't need retconning if the lore simply doesn't acknowledge them as paladins but rather as these hypothetical "Void Knights". Paladins would be the light wielding plate wearers with a beacon placing healing spec, a shield throwing tank spec, and a two-handed melee dps spec, and the hypothetical "Void Knights" would be the void wielding plate wearers with a beacon placing healing spec, a shield throwing tank spec, and a two-handed melee dps spec.
I wouldn't get too worked up over this little flight of fancy though. I don't think Blizzard would ever actually do such a thing. Especially not just to give Void Elves Paladins but "not paladins". For anything like this to happen Blizzard would have to want to open paladins up to a wider range of races overall, not just one allied race. And they'd have to come up with some compelling lore to make it work and have it make sense. I don't see any of that happening... I just think the concept of a "dark paladin/void knight" fits the "rule of cool" qualifier.