when changing the thermostat do you have to drain the coolant?

 

Nope, just pot a towel around the housing along with a towel on the optispark and youll be fine. remember to do it while its cold (like after its cooled overnight) that way only a couple drops will spill.
after you change it your going to probly have to bleed the system of air.

 

what do you mean bleed the syste?. and how do i do that?

 

well after you change the thermo if you spill enouph antifreeze , air is going to get in, so what you need to do is run the car and up the small air purge valves on top of the engine. when antifreeze starts pouring out smoothly ( not bubbly or anything but like a smooth stream) that means you have no more air.
the reason why you dont want air in your system is because the car will over heat if you do.

if you dont feel comfortable doing it this way you can just run the car with the radioator cap off until the fans come on. at that point the water level will start to raise, cap it and close it before it starts pouring out. but be carefull the water will be BURNING hot. i wouldnt recomend doing it this way. last way you can do it is run the car till the fans come on, shut it off and let it cool. then when it cools add more antifreeze to the radiatr. repeat proccess till full.

i would recomend the first way though. easiest and safest.