paperclip_migration.rake

# First configure your models to use Amazon s3 as storage option and setup the associated S3 config.
# Then add the classes your want to migrate in the klasses array below.
# Then run rake paperclip_migration:migrate_to_s3
# Should work but this is untested and may need some tweaking - but it did the job for me.

namespace :paperclip_migration do
  desc "migrate files from filesystem to s3"
  task :migrate_to_s3 => :environment do
    # Replace with your real model names. If anyone wants to this could be picked up from args or from configuration.
    klasses = [:account, :product, :product_category, :studio]
    klasses.each do |klass_key|
      puts "moving #{klass_key.to_s}"

      klass = real_klass(klass_key)
      next if !klass

      if !klass.respond_to?(:attachment_definitions)
        puts "There are no paperclip attachments defined for the class #{klass.to_s}"
        next
      end
      definitions = klass.attachment_definitions
      puts "with attachments #{definitions.keys.inspect}"

      klass.find_each(:batch_size => 100) do |record|
        definitions.keys.each do |definition|
          next if !record.send("#{definition}_file_name") # don't run if there's no file to upload
          next if record.send("#{definition}").exists? # don't run if you already uploaded
          
          attachment = Paperclip::Attachment.new(definition.to_sym, record, definitions[definition.to_sym].except(:s3_credentials, :storage))
          
          # This is where I grab the existent file url.
          attachment_url = attachment.url(attachment.default_style, false)
          attachment_path = Rails.root.to_s + "/public" + attachment_url
          
          if !File.exists?(attachment_path)
            puts "For #{klass_key.to_s} id=#{record.id}, can't find file: #{attachment_path} NOT MIGRATING..."
            next
          end
          
          file_data = File.open(attachment_path)
          puts "Saving file: #{attachment_url} to Amazon S3..."
          record.send("#{definition}").assign file_data
          record.send("#{definition}").save
        end
      end
    end
  end
  
  def real_klass(key)
    key.to_s.classify.constantize
  rescue
    puts "#{klass_key.to_s.classify} is not defined in this app."
    nil
  end
end

I added a check so that it skips attachments that have already been uploaded. That way you can run it multiple times, in case you get interrupted. Also did some cosmetic refactoring.