in-place upgrade from stock mysql 5.5 to Percona Server 5.6

Running into issue upgrading a copy of our production database. I have both the master/slave running on the same machine as this is a sand box used for testing.
some of the users might have been created a long time ago using a very old mysql client. So I have already uninstalled the mysql 5.5 on this server and installed PS 5.6, how do I workaround this issue without having to update passwords for all the old users by re-installing stock mysql?

/usr/bin/mysql_upgrade -u root -pxxxxxxxxxxxxx -S /mysql/sand1_slavedb/sand1_slavedb-sock
Warning: Using a password on the command line interface can be insecure.
Looking for ‘mysql’ as: /usr/bin/mysql
Looking for ‘mysqlcheck’ as: /usr/bin/mysqlcheck
Running ‘mysqlcheck’ with connection arguments: ‘–socket=/mysql/sand1/sand1-sock’ ‘–socket=/mysql/sand1_slavedb/sand1_slavedb-sock’
Warning: Using a password on the command line interface can be insecure.
Running ‘mysqlcheck’ with connection arguments: ‘–socket=/mysql/sand1/sand1-sock’ ‘–socket=/mysql/sand1_slavedb/sand1_slavedb-sock’
Warning: Using a password on the command line interface can be insecure.
mysql.columns_priv OK
mysql.db OK
mysql.event OK
mysql.func OK
mysql.general_log OK
mysql.help_category OK
mysql.help_keyword OK
mysql.help_relation OK
mysql.help_topic OK
mysql.host OK
mysql.innodb_index_stats OK
mysql.innodb_table_stats OK
mysql.ndb_binlog_index OK
mysql.plugin OK
mysql.proc OK
mysql.procs_priv OK
mysql.proxies_priv OK
mysql.servers OK
mysql.slave_master_info OK
mysql.slave_relay_log_info OK
mysql.slave_worker_info OK
mysql.slow_log OK
mysql.tables_priv OK
mysql.time_zone OK
mysql.time_zone_leap_second OK
mysql.time_zone_name OK
mysql.time_zone_transition OK
mysql.time_zone_transition_type OK
mysql.user OK
Running ‘mysql_fix_privilege_tables’…
Warning: Using a password on the command line interface can be insecure.
Warning 1642 Pre-4.1 password hash found. It is deprecated and will be removed in a future release. Please upgrade it to a new format.
Running ‘mysqlcheck’ with connection arguments: ‘–socket=/mysql/sand1/sand1-sock’ ‘–socket=/mysql/sand1_slavedb/sand1_slavedb-sock’
Warning: Using a password on the command line interface can be insecure.
/usr/bin/mysqlcheck: Got error: 2049: Connection using old (pre-4.1.1) authentication protocol refused (client option ‘secure_auth’ enabled) when trying to connect
FATAL ERROR: Upgrade failed

secure_auth is turned ON by default, some users created using the old password hashing algorithm were present. Rather than continuing on with the upgrade it just exits the upgrade process, will convert those old passwords to use new hashing, should be smooth next time.