Seems interesting, but it could just be an elaborate April Fool's Day prank. I doubt that Frictional has TOO much else they are working on right now that they could pull something like this.
It may very well just be a prank, but it seems like a lot of trouble to go through for something like that. There are new entities, new sounds, and new music. Surely they wouldn't go through all that trouble just to prank the minority of the game's players that will go through the files and notice the changes.