Rise of Darkness
Demotion is used, primarily, as a means to punish failure. Asmodeus can demote any devil at any time. Archdukes can demote any devil that owes them their ultimate fealty, regardless of rank. Greater Devils can demote any underling beneath them in the hierarchy, but only those that directly report to them. This same idea holds true throughout the ranks below greater devils.
Devils often fear demotion more than death. The worst demotion possible is the Nupperibos.
Some promotions into more “powerful” forms are dreaded. Though these new forms are powerful physically, they lack the mental capacity to corrupt souls which is, in general, the easiest way to gain promotions.
These lateral demotions are twofold useful. First, the need for potent powerhouses is always present in the precarious power struggle in the Nine Hells. Secondly, it strips the promoteee of their will and ambition. Above all else, these promotions cannot be contested as unlawful because they are technically promotions. Once a devil is promoted, or demoted from one of these lateral forms, they retain only vague memories of the experience. As always, devils remember those they need to take vengeance out upon.