I believe I have some good news for you. There is no need to drain the coolant in order to replace the temperature sensor. I had this job done on my Cayman last year by Porsche in Greece and there is no charge for coolant replacement (labor or spare part) in the receipt. As we all know, there is no way an official Porsche dealer is going to forget charging you for a job done or a spare part replaced on your car. The only things charged are labor for dismantling the engine cover and replacing the sensor, plus the cost of the sensor as a spare part.