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first, I don't know much, just basing on the experience I've had, changing the encoding won't be changing just one line.
some of the things you will need to change is the special characters of all the language files in the source code, and all the data stored in the database so the content is saved as the new encoding. A language expert or translator could help you more with this.
Encoding is tricky it could be simpler with the advances being done in the code, yet it be better you recieve more consulting with this... maybe with people more familiar with japanese, and translators familiar with encoding procedures.