Angels in hell. Demons in heaven.
But aesthetic choices are not arbitrary. The angels, incarnate evil now, would soon find that their bodies constructed for gentleness and healing were too thin and frail for their new purposes. Thickness of limb, stoutness of form would be the new order of the day. Fingers composed for graceful art would be repurposed as weapons, becoming long talons. Mouths would be reshapen to rend flesh and drink blood. Light would be turned into a weapon of blazing fire to blind their enemies. All moderation would be shunned. Cool would become bitter flesh burning cold. Life sustaining warmth would become flesh blistering heat. All things unnecessary, all things creative, all things expressive would be hated, shunned and destroyed. The angels of hell would begin to look like the ravished land that they dwelt in, save for those that maintained an outer veneer of beauty so as to better deceive and control those fools that would bargain with them.
Meanwhile, back in heaven, the now saintly and righteous demons would likewise look upon their raiment of flesh with increasing disdain. These flesh rending jaws, these tearing claws, and these vainglorious crowns of horn would all need to go, in favor of forms more gentle and nurturing and symbolic of things creative and not destructive. Weapons might be needed to protect the weak from the strong, but they ought to be things laid aside as necessary but unloved tools - not things to worship and love as ones own flesh. To love the sword for its beauty or the arrow for its swiftness more than the things they were meant to protect, or to desire to appear formidable and terrifying in order to dominate the will of others, these were things to be eschewed - visors and helms meant to be donned in the face of evil - not ones actual true and desired face. They would change their forms to ones better suited to their nature. Indeed, they would have a priori done so in this world, for they always would have been that way.
And, while the results and aesthetics wouldn't necessarily look exactly the same after this radical evolution as before, nonetheless an observer familiar with one would soon recognize a family resemblance between the forms in Hell in this reality and the forms in Hell in every other reality, and between the forms in Heaven and those in every other reality.
The power is not in the name. The power is in the thing named.
There is I think one critical difference. In the monster manual, the forces of good tend to be greater than the forces of evil. Metallic dragons are stronger than their chromatic counter parts. The warriors of heaven greater than the warriors of hell. In this new world order, things would be grimmer than before. The good hearted orcs clothed in their skin of life giving green wouldn't stand a chance. Only in the Underdark would good have any hope, but even then, cut off from the positive healthy power of the sun, what hope could the forces of good have?
That makes for a weird sort of topsy turvy, as vampires are now good but must avoid the sun? Is the absence of light to be a symbol of good? The absence of something has always been metaporically and literally associated with the nihilistic goals of evil. Death is the absence of life. So I'm not sure that this situation where the strongest forces of good are associated with darkness is really all that sustainable. I suspect evil in this universe would be trying to reclaim the dark, just as the forces of good would want to reclaim the light. After all, metaphorically and literally that's what they both want.
