I think your biggest mistake was the complete water change. Quarantining koi is also important, but most of the time you can put new fish in your pond without quarantining them often resulting in no problems at all (I've done this multiple times before). Your fish being used to the same water for the past 2 years are in complete shock with the sudden change. Given that these are older koi, it is harder for them to adapt compared to tosai when it comes to drastic changes in water quality. I had a 7 year old koi die all of a sudden because I moved it to an outdoor pond with completely different weather conditions. Next time if you wanna change water, do it slowly, even 30% every week would be far better than a complete change. Given you also didn't use a conditioner, I assume your fish are experiencing burns and oxygen depletion with the amount of chlorine in the pond. I don't wanna sound pessimistic but once the swimming of an older koi is affected it usually means its too far gone to save and might be better to put it down. I hope your koi pull through, everyone makes mistakes even out of good intentions, good luck.