Hold your hand through the process of converting an existing Drupal 7 file field to use the private file system.Update the $FIELD variable (and, if required, the path and arguments for drush) and do what it tells you to.

make-drupal-file-field-private.sh

#!/bin/sh
#
# This script will hold your hand during the process of converting an existing Drupal 7 file field from public to private.
# 
# http://twitter.com/thsutton
# http://www.linkedin.com/pub/thomas-sutton/55/391/350
# http://thomas-sutton.id.au/

set -eu

#
# CONFIGURATION
#

# Field machine name
FIELD="field_file_field"

# Path to drush
DRUSH="/usr/local/bin/drush"

#
# PROCESS
#

# Check that there *is* a private file system configured.
if $DRUSH vget file_private_path --exact >/dev/null 2>&1; then
 # @todo Check that the directory exists, is writable, etc.
 true
else
 echo
 echo "[error] Please configure a private file system directory." > /dev/stderr
 echo
 exit;
fi

# Drop temporary tables, if they exist.
$DRUSH sqlq "DROP TABLE IF EXISTS temp_field_data_${FIELD};"
$DRUSH sqlq "DROP TABLE IF EXISTS temp_field_revision_${FIELD};"

# Create temporary tables.
$DRUSH sqlq "CREATE TABLE temp_field_data_${FIELD} LIKE field_data_${FIELD};"
$DRUSH sqlq "CREATE TABLE temp_field_revision_${FIELD} LIKE field_revision_${FIELD};"

# Copy data into temporary tables.
$DRUSH sqlq "INSERT INTO temp_field_data_${FIELD} SELECT * FROM field_data_${FIELD};"
$DRUSH sqlq "INSERT INTO temp_field_revision_${FIELD} SELECT * FROM field_revision_${FIELD};"

# Truncate the data tables.
$DRUSH sqlq "DELETE FROM field_data_${FIELD} WHERE 1=1;"
$DRUSH sqlq "DELETE FROM field_revision_${FIELD} WHERE 1=1;"

# Download and install the module.
if $DRUSH pm-info filefield_paths 2>/dev/null | egrep '^\s+Status\s+:' | grep -qi enabled; then
 echo "filefield_paths already installed"
else
 $DRUSH dl filefield_paths
 $DRUSH en filefield_paths
fi

# Tell the user to configure the field to use private file system.
echo ; echo ; echo
echo "GO AND UPDATE ${FIELD} TO USE PRIVATE FILE SYSTEM"
echo ; echo ; echo

echo "Change made?: [Yn]"
while read complete; do
 case "$complete" in
   "y"|"Y"|"yes")
     echo 'Good!'
     break
     ;;
   *)
     echo 'Well hurry up!'
     ;;
 esac
done

# Copy data back to original tables.
$DRUSH sqlq "INSERT INTO field_data_${FIELD} SELECT * FROM temp_field_data_${FIELD};"
$DRUSH sqlq "INSERT INTO field_revision_${FIELD} SELECT * FROM temp_field_revision_${FIELD};"

# Drop the temporary tables.
$DRUSH sqlq "DROP TABLE temp_field_data_${FIELD};"
$DRUSH sqlq "DROP TABLE temp_field_revision_${FIELD};"

echo ; echo ; echo
echo "Now edit ${FIELD} again and tick the 'Retroactive update' checkbox in the 'FILE (FIELD) PATH SETTINGS' fieldset."
echo ; echo ; echo