I'll warrant the OS upgrade had nothing to do with the situation you find your text files in.

If they are just text files and not MS Word files then you should be able to open them with TextEdit.

If they are rtf documents you should still be able to open them from the "Open..." item in the TextEdit "File" menu.

Do you have one of the problem files that you are pretty certain does not contain sensitive information? Perhaps someone can examine it with your assistance and tell you what you need to do to resolve the problem.