I recently upgraded an old website[1] stuck on Rails 4.2 / Ruby 2.4 (yeah, I know) to Rails 8.1 / Ruby 4.0 and it's still awesome as it was in the first days of Rails 2.3 back in the days. Such a great framework and language to work with!
railsdiff.org was really helpful to upgrade in addition to the official documentation, especially now with LLMs that can process all these text and assist you in the process.
[1]: https://news.ycombinator.com/item?id=49587692