This allows you to migrate files on S3 stored by paperclip to a new location.

paperclip_s3_migrator.rb

# Example usage:
#
#    PaperclipS3Migrator.new(
#      [User, BlogPost, Page],
#      ':attachment/:id/:style/:filename',
#      ':class/:id/:attachment/:style/:updated_at_:filename'
#    ).migrate
#
class PaperclipS3Migrator < Struct.new(:klasses, :old_path, :new_path)

  def migrate
    klasses.each do |klass|
      KlassMigrator.new(klass, old_path, new_path).migrate
    end
  end

  private

  class KlassMigrator < Struct.new(:klass, :old_path, :new_path)

    def migrate
      model_scope.find_each do |record|
        attachments_for_klass.each do |attachment_name|
          if record.send("#{attachment_name}?")
            migrate_attachment(record.send(attachment_name))
          end
        end
      end
    end

    private

    def model_scope
      klass.unscoped.where(conditions)
    end

    def conditions
      attachments_for_klass.map {|attachment_name|
        "#{attachment_name}_file_name IS NOT NULL"
      }.join(' OR ')
    end

    def attachments_for_klass
      @attachments_for_klass ||= Paperclip::AttachmentRegistry.names_for(klass)
    end

    def migrate_attachment(attachment)
      attachment.styles.keys.push(:original).each do |style|
        StyleMigrator.new(attachment, style, old_path, new_path).migrate
      end
    end

  end

  class StyleMigrator < Struct.new(:attachment, :style, :old_path, :new_path)

    def migrate
      old_object.copy_to(new_key, {:acl => :public_read})
    rescue => exception
      puts "[PaperclipS3Migrator][#{attachment.instance.class.name}:#{attachment.instance.id}:#{attachment.name}:#{style}] #{exception.inspect}"
    end

    private

    def old_key
      attachment.send(:interpolate, old_path, style)
    end

    def new_key
      attachment.send(:interpolate, new_path, style)
    end

    def old_object
      bucket.objects[old_key]
    end

    def bucket
      s3_client.buckets[s3_credentials[:bucket]]
    end

    def s3_client
      @s3_client ||= AWS::S3.new(s3_credentials.except(:bucket))
    end

    def s3_credentials
      Paperclip::Attachment.default_options[:s3_credentials]
    end

  end

end