When coding with some framework we seldom come to a stage where we need to upgrade our framework as latest version of it is released. Upgrading your framework to latest version is not often so easy especially if there is a major version update.
With respect to rails say that you have been working on rails 3.2.12 and there has been a latest release of rails version 3.2.16 in this case there should not be any major update with your framework just few changes. This can be handled easy by using command bundle update with the version of rails you want to update to.

In another scenario suppose you have been working on rails 3.2.12 and you need to update rails 4.1.0. This seem to be a major update in framework and while updating your framework in the scenario proper care should be taken. Because there can be some major changes while moving from rails 3.2.12 to rails 4.1.0 you may even need to change your code according to new framework requirement depending on your project.

For more information regarding to rails version upgrade please follow the following link

http://crabonature.pl/posts/27-updating-ruby-on-rails-version