How to properly downgrade mysql 8 to 5.7 in centos 8
Downgrade from MySQL 8.0 to MySQL 5.7, or from a MySQL 8.0 release to a previous MySQL 8.0 release, is not supported. The only supported alternative is to restore a backup taken before upgrading. It is therefore imperative that you back up your data before starting the upgrade process.
https://dev.mysql.com/doc/refman/8.0/en/downgrading.html