MCI for Kansas City International is an interesting one because Mid-Continent was Wichita (ICT) before it was renamed Eisenhower.
Indeed, Dulles is IAD (there is no IATA code "DUL" -- even Duluth, MN, is DLH) and currently there is no airport coded DJT.
SNA (Santa Ana, CA) was renamed for John Wayne in 1979 with no effect on the airport's IATA code. Its previous name was Orange County Airport.
And thanks to all for the good info -- ORD was left unchanged in order to honor its historical name of Orchard Field. Fair enough.