I solved the issue simply by setting Hide normal windows to the original Ctrl+Super+D.
As somebody else told me, the shortcut for Workspace switcher is not customisable.
However I think I discovered a bug: setting Super+S for Hide normal windows actually sets it for Workspace switcher. Would like to report this bug but don't know which package is responsible for this behavior.
The setting is available, but is not exposed in the user interface.
You can install dconf-editor to have a graphical interface to change a plethora of settings. The settings you are looking for are
org.gnome.desktop.wm.keybindings switch-to-workspace-down ['<Super>Page_Down', '<Control><Alt>Down']
and
org.gnome.desktop.wm.keybindings switch-to-workspace-up ['<Super>Page_Up', '<Control><Alt>Up']
You can change the assigned keys using dconf-edit. The settings can also be changed through the terminal with the command gsettings, e.g. the command
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-down "['<Super>Page_Down', '<Control><Alt>Down', '<Alt>d']"
would add Alt+d as a shortcut key to switch to a workspace down.
One can reset a dconf setting to the factory default setting with gsettings reset, as in
gsettings reset org.gnome.desktop.wm.keybindings switch-to-workspace-down
Best Answer
You can use CompizConfig Settings Manager
. Go to Desktop > Expo, select Bindings tab and change the value of Expo key as you wish:
Generally, if you want to find quickly a keyboard shortcut which is set in CompizConfig Settings Manager, you can use Advanced Search option (Settings value must to be selected):