In reverse order.
You need characters with some nuances, but mostly it seems to be a change of perspective that makes this possible.
Imagine a Harry Potter fix-fic, but from, oh, Draco Malfoy's perspective instead of Harry's. Now Draco is a bigot and an asshole, so it's hard to sympathize with him, but if you have opposing but sympathetic characters in canon? Fix for one character, and look through another one's eyes.
As for the fic in question? It's a Worm one. There's a lot of space between 'better' and 'happy' there and it's not always a smooth gradient.
Back to the original question. I haven't seen one yet. I also haven't been looking for that in particular.
You need characters with some nuances, but mostly it seems to be a change of perspective that makes this possible.
Imagine a Harry Potter fix-fic, but from, oh, Draco Malfoy's perspective instead of Harry's. Now Draco is a bigot and an asshole, so it's hard to sympathize with him, but if you have opposing but sympathetic characters in canon? Fix for one character, and look through another one's eyes.
As for the fic in question? It's a Worm one. There's a lot of space between 'better' and 'happy' there and it's not always a smooth gradient.
Back to the original question. I haven't seen one yet. I also haven't been looking for that in particular.