Restarting Office means to exit all running apps. It's recommended any time you make a change that affects all of the Office apps, like changing a proofing language, that you exit all Office apps except for the one you're using to make the change, such as
Word. And to also make sure there is only one document window open and to exit the application immediately after making the change.
More on the above: In the Office apps, when you modify settings that affects the entire application, such as in File/Options, the updates to the application aren't made immediately. They're made when you exit the application. The document window you used
to make the change holds a copy of the settings. If it's not the last window you close to exit the application, then any changes you made are discarded. The reason for this is the settings "held" by the last closed document window is what is used to update
the application. The same applies to settings that affect all of the Office apps. The last closed app updates app settings. Rebooting your computer isn't necessary, but you can if you'd like. I'd actually reboot before I made changes that affect Office to
make sure every all running Office tasks are closed.
Regarding your question about the Registry, what you have is correct. The reason we recommend renaming it is so you can return to your previous settings if you like by deleting the newly created key and renaming the old one back. If you're certain you don't
want to keep any custom settings, you can delete it.
But keep in mind, Word cannot be running when you make changes to your Registry. If it is, any changes you make will be overwritten with the settings from the last document window you close and the key won't actually be rebuilt using the
defaults.