From the how-did-I-miss-this department comes textutil, a command-line tool for juggling various document formats, including HTML, Word’s .doc, RTF, and pure text. This amazing little utility is built right into Tiger, so there’s nothing to download or install. Converting files with textutil is quite simple, requiring only beginner level Terminal skills.
For example, to convert a file from one format to another, open the Terminal, ‘cd’ to the folder where the files are located, and do:
textutil -convert destination_format current_filename
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but couldn't you save all the trouble of using the terminal and simply change the file extention? What's the big deal?
In short, no. Changing the filename extension won't change the contents of the file, which is the purpose of textutil. It actually converts the contents of the file to a different format.