I've personally worked on several Rails apps that have been through Rails versions 2 up to 5. Most of the time (if you have good tests) the upgrade process is

I've personally worked on several Rails apps that have been through Rails versions 2 up to 5. Most of the time (if you have good tests) the upgrade process is fairly painless, and has been becoming easier with more recent releases. As mentioned by others, the biggest pain is usually from third party gems (e.g. I have one app I only just moved to Rails 5 because it used a non-standard database adapter that has only just released a version compatible with Rails 5). Going through your whole app at upgrade time is also an excellent excuse to have a broad look over the codebase and pay off some technical debt.