[actsasbuffoon]

The upgrades have progressively become much less painful. Rails 1 to 2 was a big deal, as the change introduced the RESTful API. Rails 3 was slightly less violent, but the asset pipeline was still a pretty big deal. Rails 4 had a lot of API cleanup/simplification. A lot of meta programming magic was removed in favor of simpler code, which resulted in some rewriting of existing code.

Rails 5 was the easiest upgrade yet. ActionCable was the big change, but it’s an addition to Rails, not a replacement for some existing tech. As a result, it doesn’t disturb much existing code.

[jeffnappi]

This has been my experience. I've been working on a large Rails project that started on Rails 4 beta in ~2014. Since then it's been incredibly stable and the upgrade paths have been quite simple from 4.0 to 4.1, then 4.2 and now we're on 5.0 with a few minor updates left before moving to 5.1.

Everything would have worked fine if we didn't keep things up to date, but just about everything in the software world needs to be continuously updated due to security updates/patches, etc.