While I managed to do the exchange-wall-against-other-wall-thing, I was still not able to figure out how backgrounds can be changed.
While it might be not that important, it would be useful for e.G. an switch on/off light event.
Another thing that I would like to know is if the background image for an overland view can be changed (without teleporting to another overland map). Maybe it might also be possible with the SET_LEVEL_ASL tool, but it is not explained in the help file...