"The column name that you entered is already in use or reserved."

One more thing we have to take care when change column name....

Error message when you try to change the name of a site column back to its original name in Windows SharePoint Services 3.0: "The column name that you entered is already in use or reserved."

http://support.microsoft.com/?scid=kb;en-us;923589&x=20&y=16