... one of the Air Bridge old-timers told me that all the floorbeams were lowered by two inches as part of the modification.
... lowering the floor beams would have been impossible as they have to attach to the fuselage sides exactly on the joint line between the upper and lower bubbles, placing them anywhere else just wouldn't work.
Well, the ABC Merchantman is in the museum at Brooklands, open to visitors. Would any engineer like to put their head in and say one way or the other ... ?