Kenpachi was in permanent shikai (stated by Kenpachi himself, by Yoruichi, and by the first databook). This is just it reaching it's true form now that he knows its name, similar to how Yumichika's shikai was that three pronged sword when he called it out with the incorrect name, and the reishi sucking vines when he called it out with the true name. Renji's bankai also got a makeover when he learned it's true name instead of a false one.
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