I'm not so sure about that, for me it was mostly just moving gems from environment.rb to a Gemfile and updating gems/plugins. After that, I fixed deprecation warnings and moved everything to AREL/scopes at a fairly leisurely pace.
Did you forget about the always-on XSS escaping? Fixing just that took me more than a week on a large project. (I still think there should have been an option to switch it off for old projects.)
