So lets say you want a bright bsp with a night sky all you would do is run lighting with lets say coag sky and then change the sky with H2gurilla to one of your darker skies.
Gotta disagree with that, the lightmaps are made for a single structure, there's checksumming involved. Once you change your structure/level the lightmaps aren't valid anymore and they will be reverted to no lightmapping.

@fletcher, i think the main things you want to alter are the direction, color and strength... diameter etc are good by default.