My advice is to change the coolant if you are replacing the water pump. So yes, drain it. Flush out the system good, you dont want to take the chance of old dirty coolant damaging the new water pump seals. Even with draining the system, likely the will be a bit of water left in the block and it may seep when you pull the pump. If you flushed the system good, only water will be remaining and will be easier to clean up if it seeps out. I generally add a can of the antifreeze makers water pump lubricant when I replace the antifreeze (Everyone has there own preference with that). Hope that thought helps; Have fun and good luck.