Make sure your ground water isn't high enough to pop your pool out of the ground. Do you have a well point pump attachment? It might not be as much of a concern if you're only draining half the pool but I would still be concerned.

Another method is to get a large sheet of plastic. Big enough to cover the whole pool with 5 to 10 feet overlapn all sides. Then you pump out the old water from the main drain while adding new water on top of the plastic. The plastic keeps the old and new water separated.

Good Luck
Mike