Usually breaking changes are highlighted in your update cycle and if something is added or slightly modified you might find a *.conf.rpmnew or similar. rpmconf [1] helps with those sorts of updates.

[1]: https://github.com/xsuchy/rpmconf

Stuff can break - there's not especially strong guarantees across versions, but if I, say, took /etc and /home from a Debian stretch system and put them on another Debian system, it would likely mostly Just Work.

It gets more exciting the further you drift - e.g. if you're going to Debian buster, or something more significantly different like Fedora/RH, you'd probably want to cherrypick config changes from /etc, though /home should be fine(tm).

etckeeper records the exact package version changes too. it's extremely handy for small shops (so unless you do 100% immutable infrastructure, it's worth it)

you start culling packages with apt purge, that should remove config files too, and see what breaks, if nothing, well, try to push your luck further.

also with dpkg -S /etc/some/file you can see which file belongs to which package and target that package

usually on a server you know what you don't need. it's a bit more trial and error on a desktop. (because those can break in a lot more fun, unexpected ways, harder to know what is needed)