Multiple Skies:
http://www.angelmapper.com/gamedev/tutorials/multipleskies.htm
As for terrain, you will need to reset all of the height maps and layers for the terrain. Not too hard but can be tedious(Just copy and pasting texture names) You will have to edit the terrain's actor properties(Right clicking on the terrain icon in map)
1.Start by copying the terrain, paste it into new map. Add Zone info and set it for terrain.
2.Import greyscale heightmaps/layers if not already loaded into editor(You can use existing mylevel if you don't want to export/import[import mostly for converting maps from other games])
3.Add the heightmap back into the terrain's default properties.
4.Add all of the layers(Greyscale not the real texture) back to the correct lines.
5.Add real textures into the correct lines and set any alignment/texture scale options.
6.Save often.
