The most important thing is to stay organized. You do not want to loose any small parts, and not all screws on the system are the same, so try to organize them logically for reassembly, preferably in seperate cases. Same goes for the other parts of the system that you have to seperate from the mainboard.
And I second the point about the ribbon cable of the topscreen. I broke one on a knackered 3DS XL attempting to get it out of it's socket. I was extremely carefull and gently used a pair of tweezers, but it broke at what was probably a stress point, where the cable went through the hinge.
It wasn't a big deal for me, since the original shell was actually broken and I wasn't gonna use it anymore unless I would have gotten it fixed (it was basically a last resort salvaging operation since the system was pretty knackered in the first place), but if you value your DS lite in it's current state, I would probably not attempt the swap.
