How to change skybox over time?

Hi,

I want to recreate the effect of a rocket launch where the skybox turns darker over time. It starts out as the regular skybox that comes with a new Unity3D scene, but over the next 20 seconds it fades to a space skybox. How would I go about doing this? I searched online for blending/fading skyboxes but I'm confused and there's a lot of different answers. I'd like to accomplish the effect through a script but if need be, animation is fine. Thanks!