Although a lot more changes to the original maggot would have been nice, i think it was about 25% that was allowed by the FAA or the type would have had to be totally re-certified. (Obviously adding to the cost/time)
The cockpit also had to maintain its original layout for crew interchangability purposes so things have to appear to operate exactly the same on both aircraft (a major selling point that Boeing was lacking in compared to the french design philosophy).
This is why the o'head panel is the same with some switches (e.g elec power) that do the same things but the way they do it is different.
Hope this helps