Does this mean everything already on the backside of the layout, including the "house on the hill", will stay as is?
Yes. There will be no change to anything that is currently on the layout. None of the existing buildings or terrain will be changed. The revisions will be between the two ends of the "window" sill. The piece of extruded pink foam will sit on the sill and extend over the north edge of the table by about one inch. The front edge of the pink foam will have a stone wall as scenery.