The first cherub didn't appear in some obscure backwater town, she appeared floating above the crowd in the middle of Time Square. To spread the transformation, she didn't need to touch or speak to anyone, laying eyes upon her was enough. To behold the divine realization that objective moral could and did exist, was more than enough to cause the transformation. And those were transformed were seen by others, and those were seen by others as well.
A pair of women fighting over 'right to life' vs 'right to chose' both became cherubs, the most shocking thing not being their change, but the realization the other wasn't evil.
A man who had spent his life thinking 'if we could just get rid of these filthy humans, the world could be so much better' however transformed into something else.
Sociopaths, sadists, psychopaths, and others who saw other human beings as vermin changed like he did.
People posting pictures on their cell phones as the change spread were the strongest vectors, millions of internet users were changed in under a minute, and then were their families, then their neighbors, and their neighbor's neighbors, there was no stopping it.
The endless surveillance that the many, MANY shadow government organizations had in place worked against them. Since most of these were populated and run by men who thought "a few corpses and 'body never found' is a trivial price for the sheeple to remain oblivious to the very real and dangerous cosmic creatures around every corner" their bases became strongholds for imps.
Pregnant women would result in the unborn child invariably becoming a cherub.
Both cherubs and imp became as sexless as dolls, not even a belly button remained. The imp's flesh was like rubber, while the cherubs were like smooth marble. They were many colors, red, blue, green, every color except those humans had been, no one could tell what 'race' others had been before the change.
The cherubs were innocent like a girl playing with her dolls. The imps were innocent like a girl using a magnifying glass on a sunny day on ants.
Their forms were varied, some wore dresses, some wore fur as small humanoid animals. Some where slim, some buff. Their halos, horns, wings, were any number of colors or materials. Some angels had wings made of crystals, other feather made of light, some wings made of metal, or wood. But all were living parts of their bodies.
Many imps and cherubs chose to simply pursue their passions, now that money and day to day survival was no longer an issue. Painting, singing, racing, boxing, dancing.
Some greedy imps conjured literal mountains of treasure for themselves, now just pretty trinkets. One cherub turned some pigeons into dodos, resurrecting the species.
The only thing anyone could give anyone anymore that they couldn't get for themselves anymore was their company. This was naturally Hell for any imps whose hunger for power over others.
Though there were difference, for impish big game hunters, they'd turn their prey into plushies for their collection, while the cherub hunters would 'capture and release'.
The only humans not changed were those of the magical ability, either themselves or their loved ones, with the wits to realize what was going on and cast the protection on themselves and their loved ones. Of course there were those who saw this was not destruction, but a transition, and embraced what came next.
The cherubs for their part didn't press the issue, perfectly willing to have the mages 'take their time', after all, they had all eternity. The imps on the other hand, while fewer in number (turns out there were more decent human beings in the world than wicked), had taken to attacking the mages as a game, after all, so what if they 'died', they'd reform somewhere else a while later.