Life Hack: Montparnasse
I advocate never changing at a train station. The walk is long, there are too many people, and it's just a hassle all around. But let's say you need to catch a train out of Paris or you need to do some shopping on Rennes. Well first things, if you're catching a train out of Paris take ligne 6 or 13 into Montparnasse. Those are the lines that are in the station. If you're just looking to go to Galleries Lafayette or need something on the high street (Rennes), ligne 12 or 4 are the lines that don't intersect under Montparnasse station. And if you're needing to make a change, wait until Raspail for line 4 or Pasteur for line 12. If you're changing from the 6 to the 13, then you won't need to walk all the way across the station because they're next to each other. But for line 4 or 12 to line 13, prepare yourself for a long walk.