But there's another thing about that isn't there? If a devil that ascended becomes just an angel, then why do angels that have fallen become known as fallen angels?
I don't really find a devil becoming a demon much like a fiend becoming a celestial. Mainly because well, despite the ideological differences of how Evil should be, they are still ultimately on the same side of Evil. Basically, Lawful to Chaotic or Chaotic to Lawful doesn't seem as big or extreme of a jump as going from Good to Evil or Evil to Good.
although, on that train of thought, what happens to a Slaad that stops being chaotic? There's a small set of fiction and space within the D&D world of Modrons going rogue and ceasing to be Lawful (while still being a Modron, mind), what about Slaads not being chaotic? Hell, they don't even have to be Lawful, considering Rogue Modrons can be any alignment other than Lawful Neutral.
Well that's a lot more chipper and funny than my idea of Good essentially being an aberration on the multiverse that goes against the natural order of things.
I do kinda prefer my idea, but your idea certainly has merit and could be a pretty good Epic Level plot hook, busting Evil's monopoly on souls and allow everything a chance at redemption is the perfect combo of silly and epic that I think all good stories need.
True, they're called fallen angels. However I believe in the case of Zariel, that although she is a fallen angel she is also a devil (in fact an archdevil). So it's quite possible that "redeemed devils" truly are a thing, but celestials (being the nice lads they are) just don't call them that, preferring to focus on the present rather than the past.
I largely don't agree on the Lawful to Chaotic jump being easier than a jump from Good to Evil (or the reverses). I'm one who believes that if the D&D multiverse is truly as balanced as Mordenkainen believes it is, than these alignment shifts are quite possible, perhaps even equally possible to each other.
So we have examples of celestials falling to evil, and of devils falling to chaos. Perhaps we could think of alignment as a sort of "degeneration," that Lawful Good is the most pure at the top, Lawful Evil and Chaotic Good are the next rung down, and Chaotic Evil are at the bottom. And that it is harder to move down a rung than to climb back up. However, this might be just the kind of propaganda that Lawful Good types would spread, that they're the "most pure."
Funny enough it is actually possible for Slaad to be lawful; they're called Gormeel. It isn't really clear if a chaotic slaad can become a gormeel, or if gormeel are just spawned accidentally from the Spawning Stone, but lawful slaad do exist.
Anyway, my larger POV is that just like for mortals, planar creatures can shift alignment. The difference is that when a planar creature shifts alignment, they're literally changing the very essence of what they are, and are evolving or changing into something else.
I'm of the opinion that yes, redeemed demons and devils do exist, but that celestials probably do their best to hide that info (or perhaps they just don't talk about it because a creature's past is irrelevant to them).