If after the upgrade the mysql doesn't start don't panic! In mysql8 mysql_upgrade command is dummy, and the database upgrades automatic on first restart. In my scenario I was upgrading from 8.0.26  to mysql8.0.27 and I see errors like bellow and the database won't start:

Today most popular storage engine in latests MySQL databases is InnoDB. But if you have older databases, before the times when InnoDB becomes the king, you must somehow convert your data. You can easy find which tables are using an older storage engine using: > SELECT TABLE_SCHEMA as DbName ,TABLE_