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DMG 57, Outer Planes

When discussing anything to do with deities, the language used must be highly metaphorical. Their actual homes aren't literally places at all, but exemplify the idea that the Outer Planes are realms of thought and spirit. As with the Elemental Planes, one can imagine the perceptible part of the Outer Planes as a border region, while extensive spiritual regions lie beyond ordinary sensory experience.

DMG 57, Outer Planes

When discussing anything to do with deities, the language used must be highly metaphorical. Their actual homes aren't literally places at all, but exemplify the idea that the Outer Planes are realms of thought and spirit. As with the Elemental Planes, one can imagine the perceptible part of the Outer Planes as a border region, while extensive spiritual regions lie beyond ordinary sensory experience.

DMG 57, Outer Planes

When discussing anything to do with deities, the language used must be highly metaphorical. Their actual homes aren't literally places at all, but exemplify the idea that the Outer Planes are realms of thought and spirit. As with the Elemental Planes, one can imagine the perceptible part of the Outer Planes as a border region, while extensive spiritual regions lie beyond ordinary sensory experience.

Alignment is to a devil what matter is to a creature of the Prime Material.

PHB p. 122

Alignment is an essential part of the nature of celestials and fiends. 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.

You might want to start your investigation of this question with a quick read on Plato and his philosophy of Ideals. Plato posited that everything we see in this world is just a shadow of a world of "ideals". When we see a dog, we see a lump of flesh attached to some bones and wrapped in fur, more or less like lots of other lumps that we also call "dogs". Plato said that since we can recognize all these peculiar lumps as "dogs", then somewhere there must be an "ideal dog" - a single creature that completely embodies all things doggy.

The DnD multiverse embraces this idea and places these ideals in the Outer Planes. The game no longer has all the summon spells it once did, but when it did, the implied mechanism was that whatever beast was summoned was not any particular beast but the Ideal of that beast. (And, yes, you read that right - the Ideal Dog is not a particular dog, it is all dogs - even those that never existed.)

Of course, to exist on the Prime Material that Ideal had to cloak itself in the stuff of that plane. Those of us made of ordinary earth can not properly perceive or interact with Ideals in their true form. This is why we have the concept of Avatars and why fiends who die on the Prime Material are not truly killed. You can't kill an Ideal through material weapons. If it is possible to truly kill an Ideal, it must be by subverting the very concept that the Ideal represents.

D&D 5e doesn't get very deep into this set of ideas, but there are a few hints here and there. You have stumbled into one of the most direct examples.

The Outer Planes are ideal manifestations of Law, Chaos, Good and Evil. Alignment is the stuff they are made of, as surely as the Prime Material is made of earth, water, air and heat. (DMG p. 59.)

All devils are made of that same material. When a devil visits the Prime Material, he manifests with a material body so he can be compatible with our elements, but this is only a "cloak" over his true nature. If you kill him, you are only rending this "cloak" - you can't touch his true nature with mere material weapons.

Finally, to the question. If you change a devil's alignment, you are literally changing the stuff of which he is made. Whether his appearance changes is subject to DM's discretion - the books don't say. But it all important ways, he will change to the celestial or fiend appropriate to the new alignment. There is at least one interesting edge case: the Neutral alignment. Perhaps a good follow-up question for this forum would be, "Is there an Ideal manifestation of Neutrality in the Outer Planes?"

Alignment is to a devil what matter is to a creature of the Prime Material.

PHB p. 122

Alignment is an essential part of the nature of celestials and fiends. 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.

You might want to start your investigation of this question with a quick read on Plato and his philosophy of Ideals. Plato posited that everything we see in this world is just a shadow of a world of "ideals". When we see a dog, we see a lump of flesh attached to some bones and wrapped in fur, more or less like lots of other lumps that we also call "dogs". Plato said that since we can recognize all these peculiar lumps as "dogs", then somewhere there must be an "ideal dog" - a single creature that completely embodies all things doggy.

The DnD multiverse embraces this idea and places these ideals in the Outer Planes. The game no longer has all the summon spells it once did, but when it did, the implied mechanism was that whatever beast was summoned was not any particular beast but the Ideal of that beast. (And, yes, you read that right - the Ideal Dog is not a particular dog, it is all dogs - even those that never existed.)

Of course, to exist on the Prime Material that Ideal had to cloak itself in the stuff of that plane. Those of us made of ordinary earth can not properly perceive or interact with Ideals in their true form. This is why we have the concept of Avatars and why fiends who die on the Prime Material are not truly killed. You can't kill an Ideal through material weapons. If it is possible to truly kill an Ideal, it must be by subverting the very concept that the Ideal represents.

D&D 5e doesn't get very deep into this set of ideas, but there are a few hints here and there. You have stumbled into one of the most direct examples.

The Outer Planes are ideal manifestations of Law, Chaos, Good and Evil. Alignment is the stuff they are made of, as surely as the Prime Material is made of earth, water, air and heat. (DMG p. 59.)

All devils are made of that same material. When a devil visits the Prime Material, he manifests with a material body so he can be compatible with our elements, but this is only a "cloak" over his true nature. If you kill him, you are only rending this "cloak" - you can't touch his true nature with mere material weapons.

If it dies outside of the Nine Hells, a devil disappears in a cloud of sulfurous smoke or dissolves into a pool of ichor, instantly returning to its home layer where it reforms at full strength. Devils that die in the Nine Hells are destroyed forever - a fate that even Asmodeous fears. (MM p. 67)

Finally, to heart of the question.

If you change a devil's alignment, you are literally changing the stuff of which he is made. Whether his appearance changes is subject to DM's discretion - the books don't say. But in all important ways, he will change into the celestial or fiend appropriate to the new alignment. There is at least one interesting edge case: the Neutral alignment. Perhaps a good follow-up question for this forum would be, "Is there an Ideal manifestation of Neutrality in the Outer Planes?"