Codex (official WordPress documentation wiki) has a list of WordPress Versions with links to changelogs and release posts for each. Though information in those changelogs can be lacking.
Note that WP doesn’t follow semantic versioning or resemblance of. Every version is a minor one and there are no major versions, so 3.9 was simply followed by 4.0 rather than 3.10. There is no significance to first digit change.
With WP’s extreme commitments to backwards compatibility there isn’t quite established update procedure. It is assumed that anything that worked on older version will continue to do so on a newer one.
Practice falls somewhere on the scale from just upgrading and getting rid of any deprecation notices (if any) to paying up to tens of thousand of dollars for complete code audit in context of upcoming release.