Yes Rich you can change the aircraft that are assigned to the flight.
Lets look at a default flight (not the acutall default flight)
Under Catagory it is 1918: First United States Airmail Route
Leg 1: Washington D.C. to Philadelphia
This flight calls for the Curtiss Jenny Military (title line from the aircraft.cfg file)
You can edit all Microsoft saved flight files ( .flt ) with any text editor but I prefer Editpad lite
Yes this is a quick plug for what I think is the best free text editor out there.
http://www.editpadlite.com/
Ok enough of that back to the real reason you created your thread
Navigate to the saved flight you want to modify. It is either in the Flight Simulator Files folder in your My Documents folder or it is in the Flights folder in the main FS9 folder. In this case it is
C:\Flightsim\FS9\Flights\airmail
Keep in mind this is my path to the folder yours will be different.
Right click the airmail01.flt file and open it with your text editor. If you want to assign the text editor as the default program it will not hurt anything.
Here is the start of the file
[Main]
Title=Leg 1: Washington D.C. to Philadelphia
Description=Re-create the first United States airmail flight from Washington D.C. (KDCA) to Philadelphia (KPNE). Estimated time to complete: 2 hours, 15 minutes.
AppVersion=9.0.30306
FlightVersion=1
What you will be looking for is the section that defines the aircraft type. It is a little over halfway down the page.
[Plane.0]
Aircraft=Curtiss Jenny Military
Just change the Aircraft= the aircraft you would like to use. Keep in mind this line come from the aircraft.cfg file.
So if we wanted to use the default C182 we would find the aircraft.cfg file
[fltsim.0]
title=Cessna Skylane 182S
And copy out Cessna Skylane 182S to the .flt file.
[Plane.0]
Aircraft=Cessna Skylane 182S
Is that all you need to do?
Don't really know there are a lot of other sections that store data about the plane. Some of those options might not be available in your chosen aircraft. All I can say is back up the .flt file before you make changes and have at it.