Okay, I'm speaking as someone who knows next to nothing about Greyhawk - but from what I've read here, it sounds like it's always been a broad strokes setting.
So if it hat magic wielders, why would Warlocks and Sorcerers be a problem beyond "they were always there, we just haven't seen them yet"?
The same goes for Goliaths or Good Orcs or Aasimar.
Back then, we played practically exclusively in The Dark Eyes world Aventuria for about two decades ... it started very broad strokes. It had humans and elves and dwarves. Orcs and goblins were monster. At some point, there was an adventure with lizard people, who where monsters. At some point, there was an adventure with humanoid undersea-creatures, who were monsters. Dwarves were anti-magical. "Demon" was the name of one monster in one adventure which looked like a Nazgul with a whip.
These days, Aventuria has at least four types of dwarves, ohne of them are is more or less a stand-in for halflings; orcs, goblins, lizard folk and undersea people are playable. Dwarves can wield magic, they even have their own type of druidic magic career (but they can be most other kinds of magic-wielders as well). Demons are a whole class of creatures with a complex mythology/theology behind them, and the "Nazgul" are among the most minor typies. Rules have changed radically, and the lore has changed along; often changs were justified with wrong in-world assumptions, so that the "objective" information of a previous edition became the misconception of the next one. Dwarves never were anti-magical, most people just didn't know about the dwarf-druids; orcs weren't naturally evil, it's just that they used to raid the humans of the sorrounding areas and therefore were painted in that light; a lot of people knew nothing about demons, so the one type they may have encountered stood for the whole class. I could go on and on ...
Still, Aventuria is considered one of the most consistent fantasy worlds ever in existence, with an ongoing history since 1986; there was never a "reset", it has evolved continuously and mostly organically. Room was made for new elements, or for new interpretations of old elements.
I don't see why something similar shouldn't work for Greyhawk.