Ratings downgraded from 4.2.1 to Rails 4.0.2

I was wondering if we are the best way to downgrade Rails from 4.2.1

to 4.0.2

. For now my plan is to remove the gemfile.lock and change

gem 'rails', 4.2.1' to gem 'rails', '4.0.2'

      

        
        
        
      

    

But I can see the problems occurring, such as the Railties

way the files are created application.rb

,development.rb

Example

Rails 4.2.1

# Development.rb
Rails.application.configure do

end

      

        
        
        
      

    

Rails 4.0.2

# Development.rb
MyApp::Application.configure do

end

      

        
        
        
      

    

Rails 4.2.1

# Application.rb
require File.expand_path('../boot', __FILE__)

require "rails"
# Pick the frameworks you want:
require "active_model/railtie"
require "active_job/railtie"
require "active_record/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_view/railtie"
require "sprockets/railtie"
# require "rails/test_unit/railtie"

Bundler.require(*Rails.groups)

      

        
        
        
      

    

Rails 4.0.2

# Application.rb
require File.expand_path('../boot', __FILE__)

# Pick the frameworks you want:
require "active_record/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "sprockets/railtie"
# require "rails/test_unit/railtie"

Bundler.require(:default, Rails.env)

      

        
        
        
      

    

Is this a big jump or is there a safe way to do it?

EDIT

So, after answering the first question I get when trying to run my application,

`require': cannot load such file -- active_job/railtie (LoadError)