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Upgrading between from 2.x to 3.x or 3.x to 4.x can tend to be tricky but I have not had many, if any, issues upgrading from 4.x to 4.2.

Typically if a functionality is removed or deprecated from the framework, you will find a gem will popup that can be included so your legacy code still functions. I often take this approach since we maintain mountains of Rails apps from 2.x to 4.x.

If you are working on a handful of apps or a single app, I think it is worth it to stay as current as possible and update with every new release. Your knowledge of the framework and the quality of the product will increase with every iteration.