Hi,  I'm a newbie (to rom hacking) so my apologies if I'm missing something obvious.

I'm trying to alter the script and character names of Uncharted Waters for the NES.   This should be the easiest hack ever, since all the text in the ROM is in pure ASCII. 

However, if I change any text (even a single character) in the ROM, it seems to screw up the entire game.   I'm using a hex editor and I'm being very careful to only overwrite existing text (and pad with spaces to match the original text length), and not to overwrite any potential null terminators. 

I was thinking it was a checksum issue, but I tried doing things like changing the word inn to jmn (i+1, n-1) so that the checksum would be the same.   But that doesn't seem to help.

Any ideas?

I'm absolutely sure.   If you have the ROM, you can try it yourself.  Changing *any* ascii value causes the same glitches.