After installing the 400 days DLC via Steam on my Mac, I was dismayed to see my saved games were gone and it was acting like a brand new installation! I had run the game the day before the DLC was available to check everything was fine and my saves were there.
So, looking at the existing threads with similar issues, I managed to restore my prefs.prop file to ~/Library/Application Support/Telltale Games/thewalkingdead/prefs.prop from my TimeMachine backup - ran the game and my saves were back! "Hooray" I thought. Also my preferred play resolution was back (2880x1800 - Retina MacBookPro)/
Then I clicked "play" on the 400 days episode - and immediately the game left fullscreen and changed to windowed mode. Uh Oh. I checked my prefs.prop file and it had changed from 62.7KB back to ~5KB. I quit the game and relaunched it, and sure enough my saves were gone again.
So it looks like I can't play 400 days using my previous choices - which kinda sucks seeing how these choices are a major selling point of the game.
If anyone has any ideas - I'd rather wait and not play this until I can do so using my previous choices.
If I find a solution I'll be sure to update here.