step_1.rb

class Foo
  class << self
    def some_method
      Bar.some_method
    end
  end
end

class Bar < Foo
  class << self
    def some_method
      # some awesome code here
    end
  end
end

step_2.rb

class Foo
  class << self
    def new(arg)
      obj = Bar.allocate
      obj.send(:initialize, arg)
      obj
    end

    def method_missing(meth, *args)
      Bar.send(meth, *args)
    end
  end
end

class Bar < Foo
  # you know it
end

step_3.rb

class Bar
end

Foo = Bar  # aww yeah