Yoinked this from eHow.
http://www.ehow.com/how_5150630_batch-change-file-extensions.html
Totally handy, for example for renaming stupid panasonic .MOD files to .mpeg for video editing.
1 Move the files you want to augment into the same folder or directory. All these files need to have the same extension. 2 Open the Notepad program and type the command line ren *.123 *456. Replace .123 with the name of the current file extension. In place of .456 type the name of the new extension. 3 Save the Notepad file as rename.bat in the same folder as the files you want to amend. Make sure the file is saved as all files instead of text documents. 4 Run the rename.bat file by double-clicking on it. This will change the file extensions.
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