Why were the F-35B lift fan doors changed after flight testing?
It was due to flow issues. Here's an excerpt form Air &amp; 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.