Is it possible to change the sky of a map ingame?

Yes it is.

In guerrila, go to "sky" and click "add". Link your skies to the skies you want to use.

Then, scroll down to your BSP's. Add another BSP, and the same BSP as your using.

Now, you'll have 0 - BSP and 1 - BSP, and 0 - Sky and 1 - Sky (if your adding 2)

0 sky goes to 0 BSP, and 1 sky goes to 1 BSP.

Hope this helps.

Something like this was achieved in coldsnap and cmt snowgrove through the time of day options. Of course, you can replace the night time with another day time, but you'll have to get the script from the maps I just mentioned and add in the widgets into the ui. Also do the above with what Nick posted and you'll get same/similar results.

Yes, you can, just do what Nick said, but also remember to do different lightmaps too, that way the BSP matches the time of day.

Ah ok. So that would make it possible to switch from the a30 sky to the swamp sky ingame. Could do some cool stuff with that ;)