Today, Microsoft has published new article about best practices and tips and tricks in upgrading process when you want to upgrade your solution to NAV2013R2 or NAV2015. After this first part about this topic, Microsoft already publish a second part with the same topic.The main reason why to read this article is because, the Classic Client is no longer supported by Microsoft. Now, it is the right time to make upgrade of your old NAV version.

Anyway, you can find awesome articles about this upgrade process here on MSDN Blog and part 2, as well. I recommend these articles.