There already is an option to show the differences when upgrading and selecting whichever version you want (or a mix of them, by manually editing the files in a shell).
On my desktop I use arch which handles it in a different way. It keeps the users configuration and saves the maintainers new version at the same place but with the suffix “.pacnew”. Instead of prompting the user, it just prints a single line of text saying that there’s a new version which is really hard to see when there’s often a hundred of uninteresting lines printed during upgrade. Has messed up things 2-3 times for me the past 8 years of using arch on two computers. In general I prefer arch, but debian handles this much better.