The format of the file has been changed. A lot of the preference files in XPress 7 and 8 are now in XML, which is more descriptive than the old format. If you open the file in a text editor, you'll see that the pairs are now wrapped in explicit tags defining the different parts or elements that make up each kerning pair.
However, I did a quick test between 6.5 and 8, and it appears that 8 can still import the old style .krn files properly, going to the issue in your first message. What sort of problem do you have when you try to import those .krn files into XPress 8?