One of the steps we recommend taking when doing an upgrade for any Rails version is to dual boot the application with your current Rails version and your next rails version.
This is important because it allows you to quickly run the test suite for both versions, having dual booting available allows you to debug and also revert to your current version in a much simpler fashion.
However, sometimes changes that you make for the new version of Rails may not be compatible with your current version of Rails. This means that you will need to use a few different techniques to get both versions to be able to use the dual booting and run smoothly.
Read more