Well it depends what you want to do.
Obviously changing the angles of the shadows and such is out of the question. What you can do is change the brightness and color of the lighting. I used to do it on my Left 4 Dead 2 map to simulate a night time to day time transition.
I have a video of it right here.
http://www.youtube.com/watch?v=qLFtn-gFH5w
This is all done with map ents.
What i did was create a targetname attribute in light_environment and gave the ent a name.
From there it acted very similar to a normal light ent.
Only problem i see here is file sizes being through the roof (Well in my case anyway).