find . -type f -name ‘[[:upper:]]*’

If you want to change files to all lower case, e.g. your mp3 music files then run the following script. The -i option for the mv command stands for interactive, i.e. it will prompt for your approval.

#!/bin/sh 
if [ $# -eq 0 ] ; 
then 
   echo Usage: $0 Files exit 0 
fi 
for f in $* ; 
  do g=`echo $f | tr "[A-Z]" "[a-z]"` 
  echo mv -i $f $g mv -i $f $g 
done

Thanks to Anil Awasare for this script