Changing display with xfconf-query

[Boris Hollas]

I've read

http://lionel.lefolgoc.net/blog/article80/xfce4-display-settings-
4-8-modes-clone-et-etendu

and tried

xfconf-query -c displays -p /Default/LVDS/Active -t bool -s 'true'

(LVDS1 is my internal screen according to xrandr -q)

but nothing happens. I use Xfce 4.8 on Debian. How can I switch displays 
programatically?