Open up Xtra. Make a SCM map. Save it. Open it in SCMdraft. Place brood war units. Save. It will tell you that theres a version compatibilty (which is supposed to happen). But wait, when its trying to get you to save it, it saves it as scm and doesn't give you the error anymore. When you open it in xtra, it then saves it properly as scx. Even if you rename it to scx, xtra doesn't THINK its an scx.
Well, I got a suggestion for when you're in that saving compatibilty thing. It should automatically detect if a map is broodwar or starcraft, and ask you if you want to resave or auto-rename (maybe even an option). On autorename, it'll automatically take the map, and save it as scm or scx, depending on the compatibilty. It will do all this without telling you