How high is high? Also give us some more details about the pond (like size) and filter.
and before doing a waterchange, test your tapwater as well.
High nitrate is not too much of an urgent problem, it points to not having enough plants and will likely cause an algae bloom. Same for high phosphate. Its not dangerous to your fish, but causes algae growth. I suspect the high phosphates, and possibly the nitrates come from your tapwater, Waterchanges wont help there.
HIgh ammonia, is an urgent problem, and depending how high it is, as well as Ph and temperature, may kill your Koi. It points to insufficient (if any?) biofilter and/or overfeeding. Stop feeding, and apply some bacteria to your biofilter. If your tapwater is essentially ammonia free, do a partial water change.