Is there a way to convert 2.7x files to 2.4x? I am doing some meshing for Fallout 3/New Vegas. For a while I would make the mesh in Blender 2.71 and then I would try to open up the same .blend mesh in Blender 2.49b (since that is what  making meshes for Fallout 3/New Vegas limits me to) to add in the collision meshes as well as export it to a NIF file, and make whatever changes I needed to in Nifskope.

When opening the mesh in Blender 2.49b, I get the message depicted below, and when I would hit okay and the file would open. Only the wireframe of the mesh would display rather than the actual mesh that should normally show up that's a shaded object.

I know 2.49b is old but is there something I could do with meshes edited in Blender 2.71 to make them usable in an older version of blender?
While a lot of new data may not translate well into 2.49 as ideasman42 explains, there is an option if you only want to transfer mesh objects.

When you use 'File->Save As' there is a 'Use Legacy Mesh Format' option that will ensure no n-gon data is saved. This allows 2.49 to open a simple mesh with face data intact.