upgrading an old rails project to the newest version

First commit anything you've done and tag it with some note about the
working version of Rails that supports that code. Next:

rake rails:freeze:edge
  (if it's really old, then just rake freeze edge)
rake rails:freeze:edge

Yes, I really meant to type that twice. The first time you update
most of the core but don't pull down ActiveResource, the next time,
the new Rake task is on your local computer so ActiveResource is added.

Then

rake rails:update

rake

And see what tests are broken.