This was my route last Monday: Todays experience when changing trains in Paris: From Gare du Nord take the rer B ligne to the south, change at Denfert-Rochereau into ligne 6 towards etoile. The 3rd stop is Montparnasse. Line 4 is not working the next days. It was easy to buy a navigo card for 7 Euro including two rides from a sales person in front of the ticket vending machine using a debit card.
-- best regards Thomas
