I am using a headless Linux server in an off-site location and I access it using `ssh`.

I tried upgrading to Ubuntu 18.04 LTS 64bit because of the constant nagging that Ubuntu 14.04 was outdated, yet I got a lot of error messages, afterwards. E.G. The mail server did not work any more.

Now, my provider offers a complete reinstall of a working Ubuntu 18.04 LTS 64bit at the expense of removing all data that was there previously.

Is there a diff-tool, that can tell me exactly what differentiates my system from an off-the-shelf Ubuntu 14.04, so I can easily backup only those files I need?