[RPG] What happens to a devil that dies in an area warded against planar travel

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.

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.

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?"

1. Plane shift. Always a popular option, if you're a mid-level planar traveler and have access to the requisite components.
2. Gate. For the party that doesn't feel like spending all day traveling.
3. Other spells or creature abilities. Most of these will be mimicking one of the first two options, so they can be lumped together. The most straightforward would be using wish to travel.
4. Planar Portals. Hard to find, but they are around. The Air Node in Princes of the Apocalypse is one such example, if you're okay with hanging out with a Prince of Elemental Evil. Classically (i.e. in older editions), you'd find these in spots that were thematically important for the plane in question, so in the midst of a tornado or permanent thunderstorm would be "good" places to look.
5. Magical Items. A cubic gate can get you there, as well as a well of many worlds. Other items can be created to do the trick, though anyone who could make such an item already has the ability to travel using methods 1-3.
6. Travel from the Ethereal Plane. Ethereal Curtains in the Deep Ethereal allow you to move to the border regions of the Plane of Air, and you can use various methods to get from the border regions to the plane proper.
7. Travel from one of the other Inner Planes. The elemental planes are all connected to each other and the Elemental Chaos, the ill-defined edge of the infinite planes. If you know the way, you can navigate from one plane to another, though certain routes take longer than others (and are quite a bit more dangerous).
8. Travel from the Astral Plane. As traveling through the Ethereal with the added step of starting in the Astral Plane and finding your way to the Ethereal from there via a color pool.
9. The Infinite Staircase. A magical stairwell connecting an infinite number of landings, each with some number of doors. Each door leads to a different plane. Though mentioned specifically as a method of travel to the Outer Planes, there's nothing that specifically prevents a door opening on the infinite cloud-filled realms of Air.