How are the aerodynamics of vehicles affected by their height above the ground? Would the same vehicle have a significantly different drag coefficient if it were lifted or lowered?
Not sure about significant, but if you lift it now all the driveline (axle, suspention, drive shaft, etc) are not tucked behind the front of the car, and are causing wind resistance. Example is look at a “normal” truck in the F150, 1500 size range. Notice that if you look under the truck not much sticks down below the front air dam. Now look at a raised truck, and you will notice that the body, and frame are higher, but the axle, etc, did not get raised so they are now in the path of the wind. So yes you have more drag with a raised vehicle.
As to lowering one, unless you end up hiding more parts, I am not sure. I have heard that the closer to the ground the vehicle is the lower the drag, but not sure why.