If the achievement says without changing the difficulty then I'd bet on most likely not.
so changing it back, then reloading the save before i changed it wont help? stupid system if thats how it works
It depends entirely on how they coded it. If it's a simple one-time binary toggle on the difficulty option, you're SoL, whereas if there's a non-zero playtime tracker on difficulty X piece to the logic, there may still be hope for you. Unfortunately, there's apparently no way to know until you finish your current playthough.
BTW, If it helps you feel less alone I'm replaying on nightmare too, and i've already lost out on platinum due to the Belle of the Ball trophy bugging for me.
it's taking me ages and it hrd as hell for me on nightmare, ill be insanly disapointed if i complete it and it doesn't pop
