It's easy to go to Hell ...
But to recall that step ... that's heavy, that's work.
--Virgil, "The Aeneiad"
Redemption is the act by which a demon renounces its selfish nature and tries to become an angel again. It's a lot harder than Falling, though -- like many things in life, it's easier to tear something down than to build it back up.
But by itself, that's not enough. A demon has to want to be something better again -- a Balseraph who wants to hear the Truth, a Djinn who wants to care, a Shedite who wants to stop hurting people and start understanding them. Sometimes a would-be pentient will manifest the Selfless Discord, indicating that they've started seriously looking outside themselves and making them a prime candidate for angelhood. And that's still not enough.
Next, the demon has to find an Archangel. And persuade them that they're worth redeeming. Neither the Archangel or its Servitors is allowed to offer any incentive or reward beyond a place in Heaven; any selfish motive will doom the process.
Some demons don't survive Redemption. Others may find themselves reduced in power. But those that make it through have become angels, and as they survive more missions and prove themselves trustworthy, they will gradually be granted the full powers and abilities that any Servitor of that Archangel would have.