There should be a drain plug on the radiator. If you are looking at the front of the car, look at the lower left side of the radiator (passenger side.) Check the front of the radiator (bumper side.) That's where mine is.
This way you shouldn't have to mess with draining the block.
However, as long as you're draining some of the coolant, it's not that much work to drain the entire block and replace the old coolant with new. Cheap insurance.