I found that Qidian seems to charge for the first published chapter by default, so subsequent modifications will be charged based on the word count of the latest chapter, regardless of whether the modified word count is more or less.

So, I decided to code the original chapter directly. I looked at the backend and found that the original chapter was 7,200 words long. I then directly increased the number of words to 8,000 words. I found that the coins were still the same and had not increased. I could continue.
Chapter completed!