In other blog posts, we've covered just about everything else regarding pond and Koi care in the winter so I thought we should cover this too.
Unfortunately, there isn't a short answer for this. It basically comes down to the climate where you live. Here in Colorado, we have a historical average range of 17F - 48F in December, but the temperature has gone above 70F occasionally. While I'm well aware that Colorado has strange weather and most places typically do not fluctuate this widely, this only proves my point that water changes in the winter are not as simple as you might have thought.
Typically, you want to stop your normal water change schedule once the water temperature in your pond gets to 46F. If you live in a more stable climate, you are probably thinking "well, that is a simple answer" and you're right, that is a simple answer. I didn't just flat out give you the answer just to make you read more though, I did it to get you to think about what the weather is really like where you live, and how many factors go into weather (humidity, the sun, clouds, snow/rain, etc).
To continue using the Colorado example, we famously get "300 days" of at least partial sun a year and this affects our weather more than anything else in the winter. Here, you could have two neighboring ponds, the only difference between the two being one is completely shaded and one is in a position to be in direct sunlight for at least a couple hours a day. Because of the heat that the sun gives it, the second pond would likely need water changes even when the first pond would not.
The best way to keep track of this is to set up a thermometer in your pond and pay attention to it when you are at home, comparing it to the actual temperature outside. If you have a long stretch of time where your water stays above 46F, you may want to consider a partial water change. Otherwise, its best to avoid water changes if possible, and only top off your pond when the water level goes down for any reason.
The last thing to keep in mind is if you have a period of three or more weeks without a water change, or without adding new water, its best to add a vitamin and mineral supplement to your pond. This will help keep your Koi healthy during their dormant period.