The key is to press up firmly on the clear lens. This will flex the long sides out where the clips are.
On some of mine you could press on the center on the clear lens, and hear a pop when one side released.
On others I had to slide a thin piece of metal (letter opener, etc) in between the cover and the ceiling and press on one end of the clear lens while twisting gently on the metal.
When that end came free I'd pull down gently with my fingertips, and do the same to the other end.
Mine are all now clear, hope this helps someone else.