I can understand the frustration of the King room becoming sold out while waiting for the response from the hotel. Certainly, I would write back and state your disappointment, and ask for an alternative solution - however, I would say that <24h is not an exceptional amount of delay in response.

Availability does fluctuate so it's worth keeping an eye on availability from time to time. Hopefully you will even find King rooms for both nights prior to your stay, particularly around the cancellation deadline (which I think was 30 days for peak periods).

For something that is time sensitive like this, it is best to call the hotel or WoH, particularly since hotels in Japan have become overwhelmed for the past year and service levels (including response times for emails) have become worse as a result. I have experienced turnaround times of several days for their reservations email at certain top-end hotels in Tokyo that are known for great service.