Considering there are two walls, thats a decrease of 2.5 inches from each wall from the interior fittings not the external fuselage dimensions. If Airbus could shave off a couple of inches per wall from their existing A330/A340 cabin to widen their seats from 18.0 to 18.5 inches, I don't see why Boeing couldn't do something similar with the 747. It should work even better on a 747, because the wall doesn't curve inward as severely as the A330. The main reason for the decrease in wall thickness in the original A350 offering was new insulation materials that are thinner than the currently used insulation.
In that link to the new 747-8I , in the video that shows the 747 morphing into the new 747-8I , does it look to you that they changed the wing with more of it swepted back ? and more wing flex ? the tail also look like it's going to get a 787 style make over.