I'm confused. (Sorry Andrew, you're dealing with a non-techie guy).
When I looked at the last modified date of the classic theme's single.php, it was from 2010 (this is before I edited and saved the file), so that means it's been a long time since it was overwritten, but I've been keeping up with the WordPress updates, and I'm always running the latest version (I use the automatic update option).
If updating WordPress to the latest version will overwrite the single.php file, wouldn't the time-stamp on it be a lot newer than 2010?
Maybe because I have purposely avoided updating the classic theme, due to having customized it and not wanting to lose my customization, that I don't have to worry about erasing any changes I made? So as long as I don't update the classic theme, I should be okay?