All written language undergoes changes, and we have just happened to see one in the character for Ki. Kanji as we know them have been through a lot of different changes, most of them due to writing and printing technology. The change from the "grain" radical to an "x" has happened not only to the word "ki" but to a lot of other characters. The Chinese have done this much more than the Japanese, I think more because their entire language is pictographic and lacks the addition of syllabary enhancers. They change the "four dots" you see under the character for "MU" (empty) into a straight line for example. I read a book where they said this was done as a response to the use of fountain pens instead of brushes, because dots are easier with a brush than they would be with a more rigid pen nib. Kind of made sense to me.