Doesn't a Mag only have 3 main parts, the body, which in this case would contain the LED, driver if there is one, switch(if not modded to a tailcap switch), heatsink and the other main parts of the light besides the reflector. If you swap the bodies, you'll end up with an empty or non LED Mag1D(presuming non LED modded 1D body).
If you are thinking about swapping the internal components of a 1D Mag P7 light to another Mag 1D body, then it should work as long as they are from the same era. It might be a bit of work though.