Question:
if everybody knows about the story, do you still have to be מקפיד what the חפץ חיים write in כלל ב אות ט that to make the story sound better or add anything is prohibited ?
Answer:
Adding anything to make the story sound worse, or to add any tidbits to the story is also loshon hora, because those tidbits were not known to the public and that is magnifying the bad. The Chofetz Chaim even says (seif 9) that you may not tell the story to someone else if he will add something to the story when he tells it over.
