Changed Fate (Demon Slayer/Kimetsu No Yaiba)

(Old title : What If?) --"You...You don't know anything of the sins I've committed since you- since you..."-- What if Koyuki was reincarnated as a demon slayer? Will this change in fate save lives, heed way to new encounters, and lead to a brilliant victory for the demon slayers? Or will it usher in a downward spiral...