A copy of garland cannot be removed completely

It seems that a copy of garland theme cannot be removed once it is created. For example, if you create a copy of garland theme and name it "NEW" and you apply the theme, then the site appears in garland theme as it should. However, if you replace theme files in "NEW" with another theme and apply the change. The site appears in broken garland theme instead of in a replaced theme.
Is there something that I am doing wrong here?