After Solas' quest with the wisdom spirit being turned into a pride demon, I really feel bad for all the demons we have to slaughter during the game. It's like we have to put down people having a violent psychotic break without having any option to try to help them.
It may depend on the circumstances of the spirit turning into a demon. Solas's friend was bound against her will, so breaking her bindings caused her to go back to Wisdom. Here's what Cole and Solas have to say on the subject:
Cole: Is there a way to save more spirits, Solas?
Solas: Not until the Veil is healed. The rifts draw spirits through, and the shock makes demons of them.
Cole: Pushing through makes you be yourself. You can hold onto the you.
Cole: Being pulled through means you don't have enough you. You become what batters you, bruises your being.
Solas: Yes, exactly. Deliberately crossing the Veil requires that a spirit form will, personality.
Solas: That concept of self gives a spirit the chance to maintain its nature.
Solas: Wrenched into this world unwillingly by the rifts, spirits suffer the same fate as my friend.
Cole: Then we will help them.
Solas doesn't seem to think it's possible to revert demons who come through rifts, or at least not while the Breach is active. Cole's last line is a bit cryptic; I wonder if by killing their physical forms and sealing the rifts, we
are helping them in some sense? They can't be restored on this side of the Veil, but if you kill a spirit, its energy just goes back to the Fade. Without the rifts, it might reform one day as a spirit and not a demon.
Why in the world, then, does Solas think that tearing down the Veil won't hurt spirits too? Maybe if it goes down all at once and the two worlds instantly merge, there isn't the battering and bruising that Cole describes of being forcibly pulled through?