Hiya! 
To make one “background” fade into another, one of them has to actually be an overlay the size of a background (ie. 640 x 1136).
It doesn’t really matter which is the overlay and which is the background, especially if it fades in and fades back out, but here’s just one way to do it:
EXT. GALAXY BACKGROUND with INSIDE WALL at layer 0
###Characters are all at layer 1 or above so all you can see behind them is “inside wall”
##Scene here
&overlay INSIDE WALL opacity 0 in 2
##Dressing game here
&overlay INSIDE WALL opacity 1 in 2
##Scene continues
(Or the other way around, where the galaxy scene is an overlay and the inside wall is the background.)