Why did the Boeing 787 design change shape from the original renderings to the actual design?
According to this answer to the same question on Quora, it was changed in order to 


  keep as many parts common with other Boeing products as possible so they could reuse old part designs, vendors and tools and let the economics of scale help bring down the costs of these parts and thus the entire airplane.


By maintaining a greater similarity to previous Boeing products, the altered designs made the plane easier to design, build and sell, for a few reasons:


Maintaining similarity with older models made the certification process much easier and faster.
Instead of having to revise, test, remodel, and debug an entirely new part, the revised design had a greater background for the engineers to work with.
Completely new parts would have required more work building the machines used to construct the aircraft. Using prior designs led to reduced costs and time required in manufacturing.


Overall, the revised design made the aircraft easier and quicker to design and build which,  considering the issues and delays suffered by even the revised model, was a necessity.