We have a significant number of customers that reinstate their upgrade protection and upgrade from old versions each month. It is a heavily tested process and customers even upgrade from 4.x and 5.x... Prior to 4.x we are a little skeptical and work with customers on those upgrades.
Be sure to look over the release notes for all subsequent versions and pay special attention to the lines that are labeled IMPORTANT. These are the items that are substantial with the release and would be the biggest changes etc. Here are the release notes. https://www.smartertools.com/smartermail/release-notes/15