Thanks for asking. It's very easy to change the planes that fly in offline missions, but you will need to download and install the "Mission Editor" for the version of Aces High II that you are currently using, and there is one inobvious "gotcha" that can cause problems unless you know about it.
First, check the version of Aces High you are using. The version number is always displayed in the upper left-hand corner of the initial sign-on screen. At the time of this writing in mid-October 2010, the current version number is 2.21 Patch 3 (often abbreviated as 2.21.3 or 2213). You may be using an older version (such as the popular 2.12.4) or a later version. Make a note.
Then, check to see if you already have the "Mission Editor" for that version. I assume you know how to start it up if you have it. Use the prominent "Help" drop-down box to select "About", which will display the version. The numbers should match (unless you are using a very old set of the software). It's important to use the proper Mission Editor version for the Aces High Version you will use to fly the mission; if they don't match, all sorts of funny things can go wrong.
If necessary, download and install the Mission Editor. You can always get the latest version (which will generally match the latest Aces High software version) from here:http://www.hitechcreations.com/download/dlteframe.html
The Mission Editor follows the usual and customary layout for Windows applications, but it's an advanced piece of software that's a bit cumbersome to use. You don't need to get very deep into it if you just want to change planes.
BUT THERE IS A GOTCHA:
Your computer must support video resolutions of AT LEAST 1280 x 1024 bits. Less that this, and you'll NEVER SEE the relevant area, because it will be hidden beneath the bottom of the displayed area! Even with 1280x1024 bits, it's very very difficult to avoid having the critical area slide beneath the visible, displayed screen surface! If you have more than 1024 vertical lines of resolution, I don't think you will have this problem. With less than 1024 lines,there's nothing you can do. It just won't work, but it won't be obvious to you. If you have exactly 1024 display lines, you can just barely get it to work, but you must NOT "Maximize" the display. Instead, you must carefully slide the Mission Editor's frame up as high as you can possibly get it by left-clicking in the top, blue bar, and then dragging the frame up so high that the top, blue section almost disappears beyond the upper edges of your physical display. You will no longer be able to see the blue bar at the top of the frame, but that's OK because you won't need it until you exit the program after you're done, and at that point you can just drag the entire frame down a bit.
Now, with the frame pushed that far up, you can left-click on the bottom right corner of the frame and drag it all the way to the bottom edge of your physical display, giving you the largest possible work area within the frame. You'll need all of that space at the bottom.
At this point, you'll be looking at a large Mission Editor frame like this one:http://TechVideoReview.com/FlightSimMovies/AcesHigh/Ah2Missions/MissionEditor/MissionEditorFrameMaximized00.JPG
The left half of your display will be dominated by the mission map, and the right half will be dominated by a list of the available missions. Choose the mission for which you want to change planes, and click on the "plus" sign associated with it. All of the "routes" within that mission will be displayed, like this:http://TechVideoReview.com/FlightSimMovies/AcesHigh/Ah2Missions/MissionEditor/MissionEditor01.JPG
Next, choose one of the "routes" for which you'd like to change the plane type. Expand it by left-clicking on the associated "plus" sign. You'll see a long list of "waypoints" associated with the selected route. It should look something like this:http://TechVideoReview.com/FlightSimMovies/AcesHigh/Ah2Missions/MissionEditor/MissionEditor02.JPG
Below the last waypoint, you should see an airplane icon and a label identifying a "Flight" of aircraft. If you have a lot of waypoints, this might be too far down your screen to see, even if you use the scrollbar. But usually you can see it if your computer is set up with high enough video resolution as described above. I've highlighted the airplane icon and "Flight" label in this image:http://TechVideoReview.com/FlightSimMovies/AcesHigh/Ah2Missions/MissionEditor/MissionEditor03.JPG
Right-click on that plane icon and, from the dialog box that pops up, select "Flight Properties". You'll see all of the details about the airplanes that make up that Flight, neatly arranged in a box that looks like this:http://TechVideoReview.com/FlightSimMovies/AcesHigh/Ah2Missions/MissionEditor/MissionEditor04.JPG