[debian-mysql] Bug#797004: Bug#797004: mysql-server 5.5.44-0+deb8u1 not installable on fresh install of Stretch

[Otto Kekäläinen]

Downgrading from MariaDB 10.0 or MySQL 5.6 back to old MySQL 5.5 isn't
supported and in many cases impossible.

If you want to do that, then purge the old installation so that there
are no configs or databases of the new format left behind, and then
install MySQL 5.5. from scratch.