We've been able to do it... for both warlocks and demons who were originally human. I wouldn't say it's a very common thing, but sometimes you can get through to them. It depends on how they ended up in the situation to begin with, like in the case of a human becoming a demon, and how determined they are to get away from being evil.
[Like how she and her sisters were able to turn back to good after getting Prue out of her marriage to an evil warlock by becoming evil themselves.]