Issues with AEM Upgrade from 6.2 to 6.3

We are upgrading our AEM from version 6.2 to 6.3. we are facing issues when we run pre-upgrade-tasks from the JMX console.

below are the steps we followed and the issues faced.
1. Took the backup of the complete instance.

2. downloaded the pre-upgrade-tasks-content-cq62-1.2.4.zip package from package share and installed.

3. we ran runAllPreUpgradeTasks() from the JMX Web Console prechecks.

4. when we ran the above we see the below information in the logs

Startup mode is RESTART, nothing to do

/var/upgrade/status/preUpgradeBackup.force does not exist, backup will not be forced

UPGRADE NOT NEEDED - StartupMode is RESTART

5. to run the instance in the upgrade mode we restarted the instance with the below command

java -jar -Xmx3072m -Dorg.apache.sling.launchpad.startupmode=update aem-author-p4502.jar

6. when we restarted the instance in the update run mode and ran runAllPreUpgradeTasks() from the JMX Web Console, we could see in the logs it is preparing for the upgrade with the below errors:

1. com.day.cq.compat.codeupgrade.impl.mergebackup.CqMergeBackupUpgrade Exception in CqMergeBackupUpgrade.run()

javax.jcr.AccessDeniedException: OakAccess0000: Access denied

2. and we see in the pre-upgrade popup msg 'disabling replication agents' and the file preUpgradeHCStatus.properties have below content

areAllPreUpgradeHealthChecksOK=false
Maintenance\ Task\ WorkflowPurgeTask=OK
Maintenance\ Task\ RevisionCleanupTask=OK
Modified\ Deprecated\ Indexes=OK
Maintenance\ Task\ com.day.cq.audit.impl.AuditLogMaintenanceTask=OK

Replication\ Agents\ Disabled=HEALTH_CHECK_ERROR

Couldn't figure out the root cause of the issue. Need help and the steps to be followed with an upgrade.

Solution:

6.3 includes pre-upgrade checks that need to pass for the upgrade to complete successfully.

You need to disable all the replication agents on your system and make sure the audit log maintenance is configured correctly (“Audit Log Purge Scheduler” in ConfigMgr) so it could be executed pre-upgrade.