The core Rails APIs are mostly the same, though you'll have to basically rewrite a couple of files in config/. I'm currently porting a moderately large applicat

The core Rails APIs are mostly the same, though you'll have to basically rewrite a couple of files in config/. I'm currently porting a moderately large application from 2.3, and few of the remaining unit test failures have anything to do with the Rails core. Your biggest problem will be that a number of third-party gems have been heavily updated for ActiveSupport 3.0, including RSpec, Devise and Mongoid, and these may cause more headaches than Rails itself. Figure a few days of updating stuff and staring at unit test failures if you rely heavily on these gems. Of course, all bets are off if you use a lot of dodgy, unmaintained gems that heavily monkey-patch Rails. We have a pretty strict policy against using dubious minor gems that stick their fingers deep inside Rails, after some bad experiences over the past couple of years.