A Brilliant, Insane Idea

It would be ironic if, because the plane that had defined them for so many eons was shifting alignment, the fiends' alignment began changing as well.

A layer in Carceri is infinitely large, therefore, it would be nearly impossible to cause the necessary change across an entire layer simply by luring celestials. Because there are at least billions (and possibly an infinite number) of Gehreleths/Demodands, in order of it to shift into, say, the Outlands, you would need billions of celestials. Otherwise, the layer's nature would stay the same and the most likely outcome would simply be the utter corruption of a huge number of Celestials. Simply put, you can't fight an infinite plane with finite numbers. This is why the Order of the Planes Millitant never gets anywhere.

That said, there's no reason they should have to try to break off an entire layer. Carceri is conveniently broken up in to spheres. These spheres are finite in size and will only have a certain, defined, number of Demodands. So all they have to is bring in enough celestials (between a few dozen and a few million, depending on the size of the sphere), and eventually that sphere will no longer match the rest of the plane and slide into the Outlands or even one of the upper planes.

Of course, perhaps the simplest way to cause part of a planar region to shift is simply by having its residents not act the way they are supposed to. This would cause a Nemausus style slip where the Layer/Sphere moved to another plane simply because its residents didn't belong there anymore. This almost certainly would not work with Gehreleths though. They are, after all, Examplars, physical embodiments of their planes. Being treacherous, sadistic monsters with no morals to speak of is just part of who they are. It's at least as natural for them as eating or breathing. Yes, it's possible for them to rise above their pettiness, but they probably won't. Which is why they need celestials.

I personally think it's a cool idea to have them lure celestials. One wonders though what will happen to the Demodands after they planeshift.

You're close, Hymneth. There are exactly 3,333 of each type of Gehreleth, ergo 9,999 in all, totaling at 10,000 if you include Apomps. They swell to double that amount in times of trouble and then almost instantly go back to the original number afterwards (and nobody knows how).

Apomps was cast out by the other baernaloths. He probably likes the idea of freedom as much as any of the titans.

Harmonium naughtiness.