If "New Game+" works like it did on PS3, your second play through has to be on the same difficulty as the last one. You can always adjust difficulty down in game.
I started my play through on HARD last night and had to step away when I realized it was 3:20 AM! Trying to get all firefly tags, collectibles, optional dialogs too but that makes it a chore. The good thing is the trophies and such will give you credit if you get all items across multiple play throughs. Hence the importance of the initial difficulty selection.
Warning: I make a lot of mistakes but occasionally I learn from them!