This is possible to do, simply copy the files, make a new virtualhost for your subdomain and it should work out of the box.
You will not be "risk free" as the database will be shared, changes you make to either site will affect the other. Things such as installing an extension will cause problems, if its not installed in both spots. Template changes and so on should be ok to test this way.
There is a much better way to do this, and thats to duplicate the site into a development site. Test in your development, without fear of affecting live.
http://www.magentocommerce.com/wiki/groups/227/moving_magento_to_another_server