gistfile1.rb

class Jay
  def talk
    puts 'hi'
  end
end

def make_say(msg, &b)
  Jay.send(:alias_method, :orig_talk, :talk)
  Jay.send(:define_method, :talk) { puts msg }
  
  begin
    yield 
  ensure
    Jay.send(:alias_method, :talk, :orig_talk)
  end
end

make_say("bye") { 
  j = Jay.new
  j.orig_talk
  j.talk
}