Offical migrating docs:
https://docs.moodle.org/37/en/Moodle_migration
If production server is running MySQL, do not, repeat DO NOT, as you say, 'upgrade' MySQL to MariaDB on this demo server.
There really isn't any advantage in running MariaDB ... it's a choice.
If the demo server doesn't have the version of MySQL you need, then use MySQL's own repo to acquire:
https://dev.mysql.com/downloads/repo/yum/
Since you are using CentOS 7, use the Red Hat Enterprise Linux 7 repo information/files from dev.mysql.com.
Now I have PM'd you my other EMail address so we can directly communicate rather than draw this out via further delayed forum exchanges.
Have also asked if it were possible to grant me ssh access to demo server. Is it possible? If so, provide info via direct email ... you have the addie.
'SoS', Ken