Whenever I upgraded to a new version of UNIX, one common problem was making sure I maintained all of the changes made to the standard release of UNIX. Previously, I did an in each directory, and then I examined the modification date. The files that were changed have an obviously newer date than the original programs [unless the changed files come from a tar archive, with their original modification dates preserved! -JP ] Even so, finding every change was tedious, as there were dozens of directories to be searched.
A better solution is to create a file as the first step in upgrading. I usually call this FirstFile. find has a that tests each file and compares the modification date to the newer file. If you then wanted to list all files in /usr that need to be saved when the operating system is upgraded, use:
find /usr -newer /usr/FirstFile -print
This could then be used to create aor file that would be restored after the upgrade.