Set-skybox-model
From FreeSpace Wiki
set-skybox-model set-skybox-model Sets the current skybox model Takes 1 argument... 1: Model filename (with .pof extension) to switch to If the model filename is set to "default" with no extension then it will switch to the mission supplied default skybox.
Comments
A very useful SEXP when you want to simulate travelling across large distances in-mission. As an example, one could use warp-effect and/or a fade-out/fade-in combo to cover up the background change, while also clearing out any area-specific objects such as jump nodes or installations during the transition.