JIRA Upgrade has not updated usernames in certain workflow Post Functions

Summary

When upgrading from JIRA < 6 to a later version, it has not updated the username in the Update Issue Post-Function from mixed-case to lower-case. Subsequently the transition fails as it cannot assign the issue to that user.

Steps to Reproduce

1. Restore the problematic data that contains this scenario.
2. Upgrade the instance.
3. Attempt to transition the workflow.

Expected Results

The issue successfully transitions through the workflow.

Actual Results

The transition fails.

Notes

Checking an XML export of the workflow we can see the value is not updated:

<arg name="field.value">CaptainPlanet</arg>
<arg name="class.name">com.atlassian.jira.workflow.function.issue.UpdateIssueFieldFunction</arg>
<arg name="field.name">assignee</arg>

Workaround

1. Backup the instance to XML.
2. Modify the XML backup to lower-case all the users in the workflows.
3. Restore the XML.