To my knowledge you don't have to actually tell anyone "Hey we will investigate your disputes now." The requirement is that you have to investigate if they give you written notice (instead of what we could do before by telling them to dispute it through the CRAs).
The caveat would be if you want to have all direct disputes directed to a central address. You have to notify your customers of that address via the CRA and another direct notification of your choosing. Most people I have talked to are not stipulating a central address for receiving direct disputes, but simply writing in their procedures that disputes can be delivered to any of the bank's branches and the branch will forward to the person/dept in charge of investigating.
I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.