As the title - the effect is immediate on changing the option. At least on my 15Khz-supporting LCD, this means the left pixels get cut off entirely with no way of getting them back (beyond I suppose using the scaler on VGA output, though I've not tried that). I don't have a real MegaSTE to see if this happens on real hardware, but this seems like an undesirable effect regardless.
As the title - the effect is immediate on changing the option. At least on my 15Khz-supporting LCD, this means the left pixels get cut off entirely with no way of getting them back (beyond I suppose using the scaler on VGA output, though I've not tried that). I don't have a real MegaSTE to see if this happens on real hardware, but this seems like an undesirable effect regardless.