Upgrading from Ruby 1.8.7 to 2.1.2 took me an hour for a medium-sized application. It involved hardly any changes except

• removing the occasional monkey patch where I had backported functionality from modern Rubies
• Migrating from require to require_relative where I loaded RSpec factories in Cucumber's env.rb (the Rails application root is no longer in the load path by default)
• replacing the old debugger with byebug
• removing sytem_timer from Gemfile (see this SO thread Show archive.org snapshot for details)
• If you get LoadError: cannot load such file -- iconv: Add gem 'iconv' to your Gemfile. iconv is a part of Ruby that was removed in 2.0. Rails 3 seems to depend on it.

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All the pain with magic # encoding: utf-8 comments is gone since UTF-8 is a default encoding in Ruby 2+.