Bran's text (Core) Reads "Plot: Kneel Bran Stark to choose and reveal a new plot card." My question is this. What happens to the other plot card? When you reveal a new one do you essentially lose the previous card and replace it with the new choice? The card says nothing about putting it back into your plot deck so to this point I've been replacing the previous with the later.

What happens when you use Bran's ability is exactly the same thing that happens every plot phase. When you reveal a new plot, the previously revealed plot moves to the used pile.