It seems the ability to easily change the Username in JIRA OnDemand was going to be in an upcoming release as stated in the following:
https://confluence.atlassian.com/pages/viewpage.action?pageId=298979005&utm_source=SAC&utm_medium=dashboard
When do you all expect this feature to be available?
As far as I know, there is no official timeline for this yet, but I would encourage you to upvote the following issue so that Atlassian knows this is something we want.
https://jira.atlassian.com/browse/AOD-7010
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