DirecTV is not DirecTV Now. It was very confusing of AT&T to name it that since there is no direct relationship between the 2 completely segregated services.
Again you can't use DirecTV sign on anymore than you can use Dish sign on for DirecTV Now since there are not the same service. There is no DirecTV Now Single Sign on at this time.I am aware. I am talking about using DirecTV Single Sign on while keeping DirecTV Now sign ins on the apps that DirecTV Now supports.
EDIT: After rereading your OP I'm confused. Are you saying you have DirecTV and DirecTV Now (at the same time)?
You should be able to sign them out and then sign in with the DTVN credentials. I also have both - couldnt say no to a free apple tv. I will be testing this out this weekend.