Cheiro, I'm using a sort of monotheistic pantheon loosely based on Tales of Wyre (imitation is the sincerest form of praise). I rely heavily on Milton for my cosmology. Thus, the demons in the abyss long pre-date the existence of Hell. Hell is formed from a piece of what was once the Abyss that The Lord of Heaven took from the Abyss to contain the fallen angels. In Book II or III of Paradise Lost, Satan leaves Hell and travels to the Abyss (right next door) and there meets Demogorgon, also known as Chaos and the creator of all. In his court is Orcus (god of the underworld--based off of the greek god Hades) and Night (the oldest of beings), out of which Chaos (Demogorgon?) was formed. Thus, IMC demons pre-date the fall. But they are also constantly created in the breeding pits. In Paradise lost, Demogorgon assists Satan in finding Earth, because like Hell, Earth and the Prime (that great sphere of worlds referred to in Paradise Lost) was created from the Abyss (which presumably is everywhere) and the elemental planes which surround the planes. Thus, while Demogorgon and crew resent the fact that part of their domain was taken from them and is now populated by devils, they hate The Lord of Heaven more for creating Hell in the first place, then adding insult to injury by creating the Prime Material Plane, in part, from the substance of the Abyss. Thus, the Abyss and Hell have combined hatred for The Lord of Heaven, and the Blood War exists as a matter of territorial expansion or reclamation.
IMC demons and the Abyss also pre-existed The Lord of Heaven. This concept, of course, is blasphemous to a follower of The Lord of Heaven. But it leaves open the possibility and probability that other gods exists. Eventually, this will become a plot point in the game. But I digress.
Because fallen angels populate only Hell, from a Miltonian perspective, it would be necessary only to compare celestials with devils to determine what they become when they fall. I like a poster's idea that it depends on the soul of the celestial. In fact, that is how it is IMC. Each devil has certain personality traits, which I try to connect with the concept of the 7 deadly sins as depicted in Dante's Inferno. A celestrial becomes the devil type that most correlates with that sin, or if no such devil exists, with the type of devil that primarily populates the layer of Hell appropriate for that sin, as depicted by Dante.