2013-06-07 : Dredge lets you replace any card draw, not just the one during your draw step.

2013-06-07 : Once you decide to replace a draw using a card’s dredge ability, that card can’t be removed from your graveyard “in response.” (Replacement effects don’t use the stack.)

2013-06-07 : You can’t use dredge unless you’re going to draw a card and the card with dredge is already in your graveyard.