Perform the following:
option 1, from CLI:
option 2, from GUI:
press <Alt>F2 to get the 'run application' dialog and type in gconf-editor and click 'run'
In gconf-editor window, go to the following key location, by expanding the keys tree:
then, double click 'screensaver' key and change the value to: <Mod4>l
and you are done. Test it by pressing super key and l.
There is also feature to search the key name and value, by pressing <Ctrl>f in gconf-editor. Actually, I found the correct location of the 'screensaver' key by searching it.