You'll need the mission's .mis file. Open it up in the M.E. Click on the Ki's mission side to open it, click & open the Ki's Route, there you will see the Ki-84 flight at the bottom of all the route info. Click on the KI84 to open up the flight properties box. Select your new replacement plane. File-Save and then File->Build res Library. All done. Really simple to do actually. Once you do it once, you'll know how to do it for any mission (but you need the mission's .mis file to open it in the M.E.!).