It shouldn't be a big issue. A little air from replacing the pump isn't worse than draining the whole system. When you button it up, run the car to operatinng temp, turn on the heat and it should purge itself of air as you drive. You aren't going to get so much air in that the car is in danger of overheating. The air will purge through the overflow and the engine will intake coolant as it cools off.

__________________
1992 300D 305+K miles
1978 240D Sold