Same as always....
For those not in a warm climate... I was just reading an old Rinko from 1983 in which there is a 'back to basics' article in a series written by Shichiro Onishi. He emphasized that 'draining the bottom water is necessary even in winter'. That is, do not skip doing water changes. The amount changed may be less than in warm seasons, but it still serves an important purpose. With the koi relatively inactive and food being withheld, water quality should be superb when Spring arrives. A clean pond during Winter sets the stage for a healthy growing season. Unfortunately, too many koikeepers take the winter off. Glarf and debris builds up in the pond, the water deteriorates, the koi have lowered immune systems... suddenly it is Spring and the fish are suffering from parasites and sores. Duh! Keeping the water should not take the winter off.