I will arrive on a Sunday evening and leave the following Sunday. I plan on getting from CDG to the Bastille station Sunday evening by way of one-way ticket on the RER-B and metro. The next morning (Monday), I plan on purchasing the Navigo Decouverte to cover the rest of the trip (Monday through Sunday- works out nicely).

My question is this: For the Sunday that I arrive, when I purchase a one-way ticket on the RER, is it good only for the train? Would it allow a transfer to the metro at Gare du Nord or Chatelet, or would I have to get a separate metro ticket at either of those stations for the transfer?