Fascinating pedantics...
ATL - Adds F terminal and a runway. Clearly new terminal. New runway. But not new airport.
DEN - Changes zip code and has entirely new concrete and buildings - New airport.
PIT - entirely new terminal, parking lots, interstate, etc., was built ~2002 and new deicing pad, etc. New airport?
DCA - (for the most part) each terminal acts independently. Terminals B, B/C, and C were segmentally knocked down and built anew. New airport?
This reminds me of a (modern) Buddhist question: if you remove your air conditioner, do you still have a car? What about the steering wheel? How much do you remove before you no longer have a car?