A cheap trick from the comand line, let's say you have a directory where the files end in MP3 instead of mp3 (lower):

kenglish@bowls:~/play/mp3$ ls
track1.MP3 track2.MP3 track3.MP3 track4.MP3 track5.MP3 track6.MP3

(first, test it)

kenglish@bowls:~/play/mp3$ for FILE in *.MP3; do NEW_FILE=`echo $FILE | sed -e s/MP3$/mp3/`; echo “mv $FILE $NEW_FILE”; done
mv track1.MP3 track1.mp3
mv track2.MP3 track2.mp3
mv track3.MP3 track3.mp3
mv track4.MP3 track4.mp3
mv track5.MP3 track5.mp3
mv track6.MP3 track6.mp3

(now, run it!)

kenglish@bowls:~/play/mp3$ for FILE in *.MP3; do NEW_FILE=`echo $FILE | sed -e s/MP3/mp3/`; echo “mv $FILE $NEW_FILE”; `mv $FILE $NEW_FILE`; done
mv track1.MP3 track1.mp3
mv track2.MP3 track2.mp3
mv track3.MP3 track3.mp3
mv track4.MP3 track4.mp3
mv track5.MP3 track5.mp3
mv track6.MP3 track6.mp3
kenglish@bowls:~/play/mp3$ ls
track1.mp3 track2.mp3 track3.mp3 track4.mp3 track5.mp3 track6.mp3

I know, for you senior admins this is a no-brainer but junior here always forgets how to do it…

note: if the file names have spaces in them, do this for the mv command, command would look like this:

kenglish@bowls:~/play/mp3$ for FILE in *.MP3; do NEW_FILE=`echo $FILE | sed -e s/MP3/mp3/`; echo “mv \”$FILE\” \”$NEW_FILE\”"; `mv “$FILE” “$NEW_FILE”`; done