There are already quite a few guides in the wild to help with the upgrade of
Rails 3.2 to Rails 4.0.
The official Rails guide
for upgrading from Rails 3.2 to 4.0 is very thorough.
With the recent release of Rails 5.0, apps currently in production running
Rails 3.2 should probably be updated to any stable Rails 4 release as soon as
There is even an e-book about
upgrading from Rails 3 to 4,
which serves as a useful guide to make this upgrade easier, and also helps
understand the advantages & disadvantages of this new (soon to be old) version.
However, if you're using any non-standard gems, you're mostly on your own. Some
gems stopped being maintained before Rails 4 was released, as was the case
with CanCan, a well known authorization
library. After many open pull requests were left unmerged,
CanCanCan was released.
It is a community driven effort to have a semi-official fork of CanCan.
It serves as a drop-in replacement for people who want to use CanCan after
upgrading to Rails 4.