refine.rb

#! /usr/bin/env ruby

module Hello
  class World
    def hi
      "wow"
    end
  end

  class Foo
    def hi
      World.new.hi
    end
  end
end

module ForceCall
  refine Hello::World do
    def hi
      "hello"
    end
  end
end

using ForceCall

p Hello::Foo.new.hi #=> expect "hello" got "wow"