I've had my headliner out several times with the seats in. There is a technique to it, and it comes out the right rear door. As Andrew says, there is a risk of damage if not done exactly correctly. Part of this is that both front seats need to be in just the right position. If you are looking at the side of the right rear door opening from outside, you need to adjust the front passenger seat to maximize the size of the opening from the top left to the bottom right (where the front seat back and 'B' pillar form a "V". In or out, make sure you have clean hands when touching the headliner (especially with the taupe interior).
Red arrow shows area where the headliner needs to pass through. Adjust seat accordingly. Help is never bad to support the headliner while it's being turned and moved.
