Just open it with AFCAD and make the change, then save the new file to Addon Scenery\Scenery. In that location, you won't even need to activate them in the Scenery Library. I've got several afcad files in that location where I've gone in and repaird things like taxiways that join up with ramps but with a huge wedge cut out.I stumbled upon this site, which I found rather interesting:
http://www.simtours.net/runwaysurfaces.php
I've noticed that a good number of real-world dirt strips in Southern California are concrete surfaces in FS2004. Does anyone know how to change the surface type in the default FS9 bgls? I think the development team got bored or lazy and decided to go with concrete in many cases rather than use information that is widely available.