I think it depends on your engine as some people on here can change the thermostat without removing anything and it seems to be on the petrol engines. I tried it with my diesel and got the thermostat off, but there was no way I could get the bottom thermostat bolt back and ended up having to jack the car up, remove the floorpan cover, move the pressure pipe, remove the drive belt and the alternator to get it back on.
I would recommend doing it properly as you can then make sure the surfaces of the engine and thermostat housing are clean, you can check for the broken prong/prongs inside the thermostat housing (almost impossible to remove the housing without damaging the prongs) and you can see everything is flush when refitting.
I had to cro-bar the alternator out and then bash it back in using a piece of wood and a mallet. I since found out you can use a vice to push the bolt retainers in and it goes in easily.
