Are you absolute sure you gave the exact filename? I just tried this on some MP3's that have been driving me crazy for ages with their "changing back to wrong tag"-behaviour. All I did to make it work was cd to the exact location, sudo and then the command given above. First I got an "Cannot open" error, double checked and found that the filename was wrong, tried again, and voilá. Finished off with tagging the files "just right" in Rhythmbox.
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