By switching two characters (コウ) with three (Kou), you probably broke one of the pointers (you know what pointers are, right?).
Except that two SJIS characters would allow him four Latin non sjis characters. So the first test he did wasn't switching two characters with three it was changing 4 bytes with 3. But otherwise I agree you should keep string lengths the same for this initial test. Some strings have there length marked somewhere and if the size doesn't match exactly you get a crash. So for latin characters add a space to the end of it "Kou " or for the sjis equivalents just use "Ko".
Some other things that might be going on:
Make sure your hex editor is set to type over and not insert. You don't want to change the file length. Your pretty much guaranteed a crash. This also means no deleting unused bytes. So if you typed in "Kou" and deleted the next unused byte you just changed the file size.
There could be a checksum in which case you are not going to edit anything in the main executable without asm hacking.