The upgrade should be pretty straight forward. Just follow the steps on https://confluence.atlassian.com/display/JIRA/Upgrading+JIRA
Could you explain a little more about the username changes?
You can follow this documentations:
My recommendation steps (but read documentations carefuly):
Thanks a lot guys. I going through this upgrade and setting up 6.3.12 in a new instance. When i am running the JIRA service, its giving me this error Windows could not start the Atlassian JIRA on Local Computer. For more information, review the System Event Log. If this is a non-Microsoft service, contact the service vendor, and refer to service-specific error code 1. I dont wee much info on error log.
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