By switching two characters (コウ) with three (Kou), you probably broke one of the pointers (you know what pointers are, right?). In the initial stages of hacking a game, we usually do tests by changing text by the same ammount of letters, just to see if we found the right text. So, try to write "Ko" instead of "Kou" and see if it changes. I believe you should look into what are pointers and pointer tables before doing any hex editing. It makes this kind of work a lot easier.
About documentation. If you already found where the text is, and it's not compressed, the same principles of hacking on most platforms apply: pointer tables, little endian, etc. The console-specific things that may give you a little headache are rebuilding the filesystem, and doing an Assembly hack.