will i have lost the staff and duriels chamber will be closed? then will i have to find all the pieces again?
You don't need to change difficulties to reset the map. You just need to make a new game. Changing the difficulty doesn't affect your quest progress in any other difficulty--it's all tracked separately.
I'm pretty sure once you use it to open the chamber on any difficulty, it remains open regardless of if you've beaten Duriel. But it's been a while.