DEM = Digital Elevation Model. It shows ground contours when zooming out sufficiently.
3D = 3D buildings and landmarks. It shows when zooming in sufficiently.
With each new map, there will be a new gmap3d.img file.
Not with each new map, there will be a new DEM file. Ground contours don't change that often, provided no major land slides have taken place.
BTW: welcome to GPS Power!