Changing the main drawing unit in the Drawing Preferences dialogue does not consistently change the coordinate display in the status bar.
I started a drawing with the application default set to centimetre and the drawing inherited this. All was well until I decided that I would prefer to work in millimeters. Ouch! Now there is no rhyme or reason as to what exactly is displayed in the status bar or (even worse) as to what scale a library object inserted into the drawing will appear.