Boeing don't have much of a choice to upgrade the screens (I can imagine a lot of the forward weight reduction comes from the choice of avionics) but that's all they'll be doing (plus a couple of long overdue changes with the engine bleed/pack function). As an A320 driver I have only ever spent a couple of hours in a 737 sim and though I was initially impressed by the level of immersion the cockpit offers a pilot I sure as hell wouldn't like the same feeling continuously for 5 hours with no where to put my feet or rest my head
I wonder what they'd find if they did a study of pilot fatigue B737 vs A320? As stated above, I'm sure cockpit comfort wasn't a major concern for the Monarch party doing the deal.