devise.rb

config.encryptor = :modsognir

irb

new.encrypted_password = old.crypted_password
new.password_salt = old.salt
new.save(false)

modsognir.rb

module Devise
  module Encryptors
    class Modsognir < Base
      def self.digest(password, stretches, salt, pepper)
        Digest::SHA1.hexdigest("#{salt}-#{password}-#{pepper}")
      end
    end
  end
end