How can I make a bsp load with a different sky?

You mean change the sky without changing the BSP in-game? If so, you can't, but there is a way to create the impression that the sky changed.

First: you need to make a copy of the BSP that you want to change skies with. In your scenario, reference this new BSP. You'll be editing the copy for the sky change.

Second: in your scenario, make a reference to the new sky as you would normally. Remember which index that sky is (0,1,2 etc.)

Third: You're going to need Kornmann's Guerilla for this. I don't know if there's any other way of doing this, so if anyone wants to show a less lengthy method please correct me. The way BSP's refer to skies is not by matching the BSP index to the sky index, but just by a reference to a sky index from each and every cluster in the BSP tag. So, prepare yourself, this might take a while, depending on how many clusters the BSP has.

Go down to the "Clusters" section of the copied BSP tag you made recently, and look for the "sky" value just under it. Change this value to the index of the sky tag from the scenario that you remembered from the second step. And here comes the lengthy part: repeat for each cluster in the drop-down list.

Then, whenever you want the sky to change, just use a simple "switch_bsp" command in your scripts and it should work out.

It seems like a lot, but it'll work. Just remember that it's still a different BSP, so you'll have to handle any AI issues that come up because of a BSP change.

Also, you don't need to do this, but you should run lightmaps on the new BSP with the new sky, since the BSP change gives you that opportunity. It tends to look strange if you use a new sky on a BSP with lightmaps made using another sky. Don't do this if you think it looks fine just with a sky change though. Just a suggestion.

i thought all u do is to go to the scenario file in guerrilla and just change the sky box

That's if you want everything in the level to have the same sky.
If you want to change the sky in game, you have to use his method. It's long, but I didn't even know it was possible. This will be usefull.

No, i meant when you load a different BSP, you load a different sky.