Note: This relates GNOME Shell 3.10 in Ubuntu 14.04. Other versions may differ and YMMV.
The keybindings of GNOME Shell can be changed using the system settings windows (All settings | Keyboard | Shortcuts). However, I have at times run into the feeling that these settings were only superficially customizable. Some keybindings would keep working the old way despite having their functionality set to another keybinding and some would seemingly ignore being disabled. GNOME keybindings seemed haunted by a ghost.
I’ve tracked down the ghost and the name of it is multiple keybindings. One functionality is often given to multiple key combos in the bowels of the system, not just one. The System settings window, however, only shows one, the first in the list. This means that if the keybinding you wish to assign to functionality A is secretly assigned to functionality B , the secret binding may overrule your attempts to use the keybinding. Only you won’t be asked to reassign the keybinding in system settings as that only checks the first, user-editable keybinding. A bug or a feature? You never really know with the GNOME development team.