Do you have to drain your entire coolant system when changing the thermostat on 2012 Honda Civic LX?

There is coolant in the hose AND in the engine.  If you lift the hose up, you will next be confronted with where the hose was removed from.  Coolant will drain from that place until the coolant level is at the lowest part of the opening. 

Please do this with a cool engine, as hot coolant can cook your flesh! 

We car repair technicians usually drain coolant into a pan before we attempt to remove a thermostat housing. 

You do not have to but you need to bleed the cooling system.

If you're not going to drain the cooling system a bit into a clean drain pan and are just going to let the coolant run out on the floor go for it and make a mess jojo. I'll bet you don't even need a thermostat and are just knobbing around trying to play mechanic!

As the car is 9 years old it would be a good plan to change the coolant anyway. Coolant gradually loses it's anti-corrosive properties over time. The result is a perforated radiator or heater matrix. Remove the old stat and flush the system through with a garden hose. Refill with fresh coolant and peace of mind occurs.

No, just open the cap to vent, drain off a quart or so the thermostat is pretty high and liquid seeks the lowest place. If it starts dripping when you loosen the retaining bolts let out a little more fluid.