I don't know, but I have a suggestion. I did a quick test, and I can open Display Properties on my secondary screen and make it primary, so I don't know why you can't. But can you boot laptop with screen closed? It may detect external monitor and use it?
Or there may be a function key on your keyboard for cycling between LCD / CRT / both. One or two presses of that could solve the issue. Perhaps fn - F4? EDIT - cycle it until the laptop screen is turned off and only the external screen is active. It will have to be primary.