The pre-upgrade checker tool checks for potential business logic issues and other problems of upgrading to Plesk 12.5, and returns recommendations that will help you solve the problems. It runs automatically as part of Plesk Installer when you upgrade Plesk. You can also download and run the checker manually before you start the upgrade operation. We strongly recommend that you run this tool.
You can download the pre-upgrade checker and find the descriptions of the report messages at https://support.plesk.com/hc/en-us/articles/213371529.
Note: When you upgrade Plesk from Virtuozzo templates, no pre-upgrade checker report appears. However, the upgrade issues might exist.