/var/lib/mysql/

Hello,

On Ubuntu 19.04, which uses packages mariadb-server 10.3.17 and
mysql-server 5.7.27, I noticed that if I wanted to switch from MySQL to
MariaDB, the database is corrupted and there is a complete data loss
even if I switch back to MySQL.
In the previous version of Ubuntu, switching from MySQL to MariaDB did
not manage to import data automatically (unlike Debian) but at least it
created a backup of the data in /var/lib/mysql-5.7/ folder which is not
done anymore.

Here is the error message I saw during install when trying to use the
database corrupted by MariaDB and switching back to MySQL:
"MySQL has been frozen to prevent damage to your system. Please see
/etc/mysql/FROZEN for help."

And in /var/log/mysql/error.log:
"[ERROR] InnoDB: Unsupported redo log format. The redo log was created
with MariaDB 10.3.17. Please follow the instructions at
http://dev.mysql.com/doc/refman/5.7/en/upgrading-downgrading.html"

I was able to reproduce the issue with a clean installation of Ubuntu
19.04 in VirtualBox.

Do you know where the problem comes from and if it is possible to fix
the binary data from /var/lib/mysql/ to make it work with either MySQL
or MariaDB?
It looks like MariaDB tried to convert the data ("[ERROR] InnoDB:
Unsupported redo log format") but now it fails with both MySQL and MariaDB.
Is it possible to revert the changes done by MariaDB to make the data
work again with MySQL? (and then do a proper backup with mysqldump)

Thanks.