Slávek Banko composed on 2017-07-27 03:18 (UTC+0200):
I tested the upgrade from Ubuntu 12.04 to Debian 7.x (Wheezy) => it was complicated and the result was not good. Likewise, upgrade from Ubuntu 12.04 to Debian 8.x (Jessie) => problems were less, but the result was also not good. There were still some Ubuntu configuration artefacts that caused problems.
I would never have tried or suggested either, because of a fundamental difference to Debian, Ubuntu's upstart init system in 12.04, which remained until 15.04.