I only ever worked on the 200 & 300 '47s but my guess is that the difference in the interiors is that the older aircraft have conventional (blue water) toilets while the 400 has vacuum type (like the '67). This gives the airline more flexibility on loo location and thus cabin layout in general. Hence Air France when changing the cabin between normal & "Vacances" (think British Inclusive Tours with a smaller seat pitch) or JAL moving from 747SR to Overseas would not have the structural changes previously required.