I scored some MS Light 400W MH floodligths. They are the common modern HID floodlight design, same as :

http://www.made-in-china.com/showroom/hellen566/product-detailtoFJDYdyfXhU/China-Outdoor-400W-Metal-Halide-Floodlight-IP65-.html

They are about 10..15 years old

The lamp cover is closed with 2 screws on the front, and the gear cover with 4 screws on the back. The screws are M5 thread or similar and are made of supposedly Galvanised Steel. The body is pressure cast Aluminum and fairly thin walled. The lamp cover screws and 2 of the gear screws go into a through hole (the end of the screw is visible from the other side of the luminaire), and the other 2 gear cover screws into a dead end hole

Some screws are rusted stuck - about 1 or 2 per luminaire. I dosed with WD-40, used big screwdriver.. The screwdriver slips out of the screw before it would turn

How to remove them in such way that does not damage the luminaire body ?

Heat the screw with a torch, as dumb as this sounds it works try tightening them a little bit and then remove them, or you could use an impact screwdriver (if you have an auto parts store close they sometimes offer free rental of tools, they just charge a security deposit) Or you could drill them out and just re-tap the hole slightly larger, and when you get them out make sure you use anti-seize/copper coat or silicone grease on the screw threads so you don't have this issue again later, aluminium is bad for this.. 

Heat the screw with a torch, as dumb as this sounds it works try tightening them a little bit and then remove them, or you could use an impact screwdriver (if you have an auto parts store close they sometimes offer free rental of tools, they just charge a security deposit) Or you could drill them out and just re-tap the hole slightly larger, and when you get them out make sure you use anti-seize/copper coat or silicone grease on the screw threads so you don't have this issue again later, aluminium is bad for this.. 

Yes definitely grease them well, it always annoys me when manufacturers don't grease screws, then a few years later, they are stuck shut.

Ash, any luck getting them out ?

Sorta busy now, have not really put much time into that. I will update when i do something

In the meanwhile the 4 lanterns are just laying outside and from 3 i removed the lamps (in the 4th there is a stuck screw on the lamp compartment) - 1 400W Pulse Start Halide, 1 400W Probe Start Halide and 1 400W SON-T. The remaining one have 400W Probe Start Halide. Which means most of them were running wrong lamps as the gear looks like 400W Pulse Start