I have had no problem changing settings i.e. adding the drawing toolbar in Word, and the contents of toolbars in both i.e I have added Change Case in Word and Paste Special in Excel and the changes have held automatically. I can only think that you have a fault in either your OS or your Office installation which is causing you a problem.
This document click here
explains how to back up your settings in MS Office, so you might find a clue here.