I mapped Ctrl-Alt-Up/Down to open web-browser and email client but it didn't take effect. Ctrl-Alt-Up/Down still switch workspaces as the default setting of GNOME 3.8.
I have tried log-out/log-in. No good.
The new keybinding you define in 'Settings-->Keyboard-->Shortcuts-->Navigation' will get appended to the previous/default one.
It will become, for example.
gsettings get org.gnome.desktop.wm.keybindings switch-to-workspace-down
['<Primary><Shift>Down', '<Control><Alt>Down']
I managed to remove the default keybinding using the following commands
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-down "['']"
gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-up "['']"
This clears all defined shortcuts for those tasks. And then just use 'Settings-->Keyboard-->Shortcuts-->Navigation' to define the desired shortcuts.
Source : https://bugzilla.redhat.com/show_bug.cgi?id=982695
