Does Plesk Onyx support dist-upgrade on Debian-based operating systems?
Yes, it does. Starting with Plesk 12.5, it is possible to upgrade the operating system on a Plesk server. The following dist-upgrade paths are supported in Plesk:
Debian 6 to Debian 7
Debian 7 to Debian 8
Ubuntu 12.04 to Ubuntu 14.04
Ubuntu 14.04 to Ubuntu 16.04 (supported by Plesk Onyx only)