In your dimisions dialog box their is a tab called alternate units click display alternate units then put in 25.4 in your multiplyier for alternate units percision to whatever you want and decimal is standard to go at least 4 places!
Changing the units is not a correct way although it is more convenient. It only changes the units you view.
You may experience problem if you export the drawings into other formats, such as: dxf.
Besides of scaling the whole drawing, you may also copy all the entities into a new drawing which use metric system as template.
However, you may need to manual copy some of the unused blocks and layers to the new drawings.