You could try Name Mangler - http://manytricks.com/namemangler/
- just download their trial version
or Automator can do it if you want to do it that way. Select Files & Folders from the Library on the left, drag Ask for Finder Items, then drag Rename Finder Items and choose Change Case, Extension Only and to: lower case. But I'd still check out Name Changer to see what it can do.