NB, I said: "If you're >= Rails 3, almost all upgrades are painless. " I was talking about upgrades past Rails 2 -> 3. That was probably the biggest one in Rai

NB, I said: "If you're >= Rails 3, almost all upgrades are painless. " I was talking about upgrades past Rails 2 -> 3. That was probably the biggest one in Rails history, but even that was a problem you should have only encountered almost 3 years ago by now. 3 -> 4 is not nearly as crazy. Recently one of the more senior devs at a place I worked at coverted a 50k line codebase from 3 -> 4 in one day. Not bad.