I'm not sure about changing hot spot coordinates, though IIRC there is a table for them, but I do know that overriding the built in (or MOD provided) interface is simply a matter of placing files with the same names in the data\interface folder. The default interface uses 1567 different files, including images and animations. And most of those files, including animations, have seperate high res versions in the high res vp. Doing a main hall "reskin" requires more work then just dropping a file into the interface folder. The anis and various other buttons used in the main hall have to match the stuff shown on the background pic. The interface also includes all the anis for command briefings, anis used on the HUD for radar/target box static (EMP nebulas) the credits images, briefing icons, not to mention all the medals. In short, making a real "total conversion" is a mamoth task. (Especially since a real TC would include all the HUD stuff too.)