Your old app does not work anymore after you have deployed a new app with new Ruby version?
This is likely that your server (Passenger) is trying to run the old app with the wrong Ruby interpreter. By default, if you do not specify a Ruby version of your app, Passenger will run the app using the global ruby version. To prevent this from happening, create a ".ruby-version" file in your application root and specify the Version in the file.
In this case, Passenger will be able to tell which Ruby version to use. There are some additional benefits creating this file, for example, Ruby IDE such as Rubymine respect the Ruby Version defined in this file.
vi .ruby-version 2.4.2