The 500 source claims the next prototype will have the roof line raised 1.5-2 inches. If that's true that's a pretty radical change it they intend on putting out cars a year from now.
I have heard this from several reliable sources and do expect it to be true as it allows for even better driver sight lines and more headroom, particularly for the passenger, as passenger headroom has been disappointing in the P4 vs the P3. This is a major structural change as it changes the frame of the vehicle however the prototype stage we are in now is exactly the time to make these kinds of changes. The chassis design must be locked down before the next step which is the testing phase where pre-production units will be thoroughly drive tested and even crashed to ensure the vehicle meets all the engineering requirements. The testing phase is also where everything is tweaked for reliability and comfort which includes things such as suspension tuning, stability system programming, engine/transmission ECU programming, and final interior fitting.