While I was doing MySQL server up-gradation on One of RHEL 6.2 server,as it is still using MySQL 5.1 version. I could able to remove the existing MySQL version easily through yum command and installed MySQL 5.5 server from RPM.
As soon as I started the server for the first times, it won’t started up and after checking the MySQL error log file ( /var/log/mysqld.log), it is found that showing Fatal error: Can’t open and lock privilege tables: Table ‘mysql.host’ doesn’t exist
solution : remove the existing mysql data directory and initialize new one using MySQL command line tool itself. Earlier I user manual steps, creating, changing permissions etc. But this command fairly easy to create new MySQL init folders with dummy data.
I executed it twice as I got some error for the first time use. Hope this would help you.