According to PHB 122, in the section entitled Alignment in the Multiverse, for most creatures, “alignment is a moral choice.” Myth says good-aligned gods created the races that are free to choose to be evil (they have free will), while evil-aligned gods created the races that are very inclined towards evil, since they were born to serve the evil god in the first place.
However, there are some types of creatures — like devils — which embody an alignment in its essence:
Alignment in the Multiverse, PHB 122
A devil does not choose to be lawful evil, and it doesn’t tend toward lawful evil, but rather it is lawful evil in its essence. If it somehow ceased to be lawful evil, it would cease to be a devil.
My first thought was to not take this reading literally. The text could have meant that a neutral evil devil “isn’t really a devil,” but really, it’s still a devil. It’s a bit like imagining a guitarist who loses both hands. The loss of some part of a person’s essence makes them not the same person anymore.
However, keeping my attention on it, I’m finding this text more and more difficult to interpret. If a devil “is lawful evil in its essence” then what does it mean when “it would cease to be a devil”? What happens to a devil when its alignment is forcibly changed?
There are ways to forcibly change someone’s alignment, and a devil can be subject to some of them.