Yes, moving the curtain is indeed a trivial operation done when the curtain is in the wrong place. The size of the J cabin can be different from the plane's last flight or the cleaning staff might have forgotten to do it.
However, my understanding is that the size of the cabin is not changed in the DCS, thus resulting in op-ups. I guess it can be done if there's a real overbooking situation which can't be helped with op-ups.