Similarly, Neverwinter Nights: Hordes of the Underdark has a half-demon (tiefling) major character who is well on the way to being completely reformed when you meet him. If you bring him along, events happen which try his strength of will, and depending on how you treat him, he may or may not slip back into his old ways.
Seems a shame when it’s a succubus who’s given up her vocation. The world could definitely use more succubi. 
Speaking of Piers, there is a reformed demon in his story “Being A Green Mother” from the Incarnations series.
The succubus Jezebel decides she wants to settle down with just one guy instead of seducing men in their sleep for all eternity.
There was also For Love of Evil which casts Satan as a good guy. Not quite a reformed demon, though - he starts as a (relatively) good human and becomes Satan later.
He was never a demon, so I don’t think that really counts. Yes, he ruled Hell for centuries but Parry was a mortal.
Fair enough, although it was still my first introduction to bad-guys-as-good-guys so i had to mention it. Incidentally, in looking for the title, I saw that Piers Anthony wrote an eighth book in the series?!? In 2007?!? Man, he is weird.
Back to the OP, there’s a borderline case in The Dresden Files (White Knight, I think). It’s actually only a shadow (think voice-in-the-head) of a demon, but apparently redeems itself and saves Dresden.