Just re use the coolant, no reason to throw it away. How old is that water pump? If you are going through the trouble of taking it off go ahead and put in a new one.
idk how old it is ive never changes it and i have put 60k miles on it, and i lost all the new coolant, due to the leaky pump, i thought it was just the rad leakn but apperently its both! it has good flow tho! so idk