A few thoughts to start with (not yet a complete answer):
- You could have a day skysphere and a night skysphere, and set one visible, and the other invisible. Swap their visibility going through the portal. That still doesn’t solve the problem of varying the lighting though…
- You could duplicate the skysphere and level layout, and set them really far apart, so the skyspheres don’t touch, and teleport the character between when entering the portal. One would be the day layout, and one would be the night layout.
- It may be possible to edit the parameters of the single skysphere, fog, etc… to transition from day and night.
It might be possible to show a video on the portal of the view on the other side from your character perspective with approach 2, but not sure how this would be done… Hopefully someone else has further insight here…