Why were the F-35's lift fan doors redesigned from those used during flight tests? The original design had two main doors which opened to each side, rather than the “car hood” style door used in production F-35B aircraft.
It was due to flow issues. Here's an excerpt form Air & Space Magazine
We discovered a few things that would need to change before the aircraft entered production. On the X-35B’s STOVL variant, the doors above the lift fan had a bi-fold arrangement: They folded and slid outward, creating an opening for air to enter the fan. However, when the airplane was slowed to a hover, the air flowing across the top of the aircraft would not on its own make the turn into the fan efficiently; it needed to be guided. On the production F-35B there is a different arrangement: Instead of two doors sliding open, a single door, hinged aft, lifts up and acts like an air scoop, helping to funnel the air into the fan. But the new design of the lift fan cover created additional challenges for the two auxiliary air inlet doors that sit right behind it. Not all the air is channeled into the fan, and at higher speeds, the air that flows around its raised cover is turbulent, causing the open doors on the auxiliary air inlets to vibrate. The hinges on those doors, therefore, had to be strengthened.
You'll also note that the Auxiliary Lift Fan doors changed (bigger, hinged on outboard edges) as a result of the main door change.