Just to let you know, you don't have to re-fill all the faces when going back from 2.6x to 2.49. Just choose save as, and on the left hand panel choose 'legacy file format', or something along those lines, and then you can open it up in 2.49 no problem unless you have some crazy complex animations, which I would doubt as they aren't required for X-Plane purposes. The change came with the implementation of BMesh into Blender, which allows for faces with more that 4 vertices. So if you have faces with more than 4 vertices you will probably have to fill these in, but you shouldn't have these sorts of faces in your finished mesh, they are more there to speed up modelling. This way you can model in 2.69, as I do as I prefer the new interface, the added tools, and because I use Blender for more than just X-Plane. Then once you're finished modelling (and texturing as well if you want to do that in 2.69), save as as above, set up the animations in 2.49 as you need the scripts to do it, then export from there.