Steps to upgrade MySQL from version 5.0 to version 5.5
Step 01 : Run this script to Dump Everything from MySQL 5.0

cd /root
MYSQL_CONN="-uroot -p..."
SQLSTMT="SELECT schema_name FROM information_schema.schemata WHERE"
SQLSTMT="${SQLSTMT} WHERE schema_name NOT IN"
SQLSTMT="${SQLSTMT} ('information_schema','mysql','performance_schema')"
MYSQL_OPTIONS="--skip-column-names -A"
mysql ${MYSQL_CONN} ${MYSQL_OPTIONS} -e"${SQLSTMT}" &gt; /tmp/dblist.txt
DB_OPTIONS="--databases"
for DB in `cat /tmp/dblist.txt` ; do DB_OPTIONS="${DB_OPTIONS} ${DB}"
MYSQLDUMP_OPTIONS="--routines --triggers ${DB_OPTIONS}"
mysqldump ${MYSQL_CONN} ${MYSQLDUMP_OPTIONS} &gt; MySQLData.sql
SQLSTMT="SELECT CONCAT('SHOW GRANTS FOR ''',user,'''"
SQLSTMT="${SQLSTMT}@''',host,''';') FROM mysql.user WHERE user&lt;&gt;''"
mysql ${MYSQL_CONN} ${MYSQL_OPTIONS} -e"${SQLSTMT}" &gt; GetGrants.sql
mysql ${MYSQL_CONN} ${MYSQL_OPTIONS} &lt; /root/GetGrants.sql | sed 's/$/;/g' &gt; Grants.sql
rm -f GetGrants.sql


Step 02 : Backup config file

cp /etc/my.cnf /etc/my.cnf.50


Step 03 : Backup /var/lib/mysql

mv /var/lib/mysql /var/lib/mysql50


Step 04 : Uninstall MySQL 5.0

Step 05 : Install MySQL 5.5

Step 06 : service mysql start

Step 07 : Login to mysql

mysql -uroot


Step 08 : At the mysql prompt enter the following:

mysql&gt; source /root/Grants.sql
mysql&gt; source /root/MySQLData.sql


That's it.

Give it a Try !!!

I have suggested this before : MySQL upgrade 5.0.88 to latest