It's a good idea to replace a 50% water change in the fall, before the water temperature falls below 60 degrees. After this point, removing and adding new water to a pond can be dangerous. Rapid temperature changes can be harmful to fish. During the winter time, fish settle at the bottom of the pond where the warmest water congregates. Adding new water will disrupt this balance of warmer and cooler water, and can mess up your fish's state of hibernation.
Instead, check your water temperature in the last few weeks of fall. Once your water is on it's way to falling below 60 degrees, go ahead and change about 50% of your water with fresh water. This will provide a clean start for your pond to last through the winter with.
