An example of how a class can be replaced with a modified version within a name space.

overwrite_class.rb

class Thing
  def colour
    'blue'
  end

  def who
    self.class.to_s
  end

  def output(line)
    puts '--------------------'
    puts "#{who} on #{line}"
    puts colour
  end
end

module MyExtender
  class OtherThing < Thing
    def colour
      'red'
    end
  end

  OldThing = Thing
  self.const_set :Thing, OtherThing

end

module MySpace
	
  t = Thing.new
  t.output(__LINE__)
	
  include MyExtender
	
  t = Thing.new
  t.output(__LINE__)

  t = OldThing.new
  t.output(__LINE__)

end

t = Thing.new
t.output(__LINE__)

This outputs:

--------------------
Thing on 32
blue
--------------------
MyExtender::OtherThing on 37
red
--------------------
Thing on 40
blue
--------------------
Thing on 45
blue