Man I devoured Hardy Boys books when I was a kid. One time I came across a much older version of one I had, so I read it, and that's how I learned they rewrite the stories every so often. It had common elements, but was a different story overall.
Then they came out with the "Hardy Boys Casefiles", the first book of which had Joe's girlfriend Iola killed by a car bomb intended for the boys. Obviously for a bit older audience.