In "Stretch", the default MySQL variant is now MariaDB. The replacement of packages for MySQL 5.5 or 5.6 by the MariaDB 10.1 variant will happen automatically upon upgrade.
$ mysql
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 32
Server version: 10.1.26-MariaDB-0+deb9u1 Debian 9.1
Copyright (c) 2000, 2017, Oracle, MariaDB Corporation Ab and others.
$ mysql --version
mysql Ver 15.1 Distrib 10.1.26-MariaDB, for debian-linux-gnu (i686) using readline 5.2
It is pretty clear to anybody ever run the tool that it is MariaDB.
In "Stretch", the default MySQL variant is now MariaDB. The replacement of packages for MySQL 5.5 or 5.6 by the MariaDB 10.1 variant will happen automatically upon upgrade.