The code changes in fpl.go update the Flight Plan (FPL) structure fields. The FlightNumber, AircraftID, SSRModeAndCode, CruisingSpeedAndLevel, DepartureAirport, DepartureTime, Route, DestinationAndTotalTime, AlternateAirport, EstimatedArrivalTime, SELCALCode, and EstimatedElapsedTime fields are modified to reflect the new values. This refactor ensures accurate representation of the FPL data and improves the overall functionality of the code.