Earlier quoted context omitted.
What is the claim? In the UK at least SCC requires that your claiming for some kind of (small) financial harm. What's that here? Even if that's not the case in the USA (small civil claims against un-pursued crime?) it's still not clear to me what the issue is, or what 'right' they've violated. The judicial system isn't there for 'how displeased you are'.
Maybe I have my github account printed on business cards or even plastered on my car as advertising for my consulting practice. Or its embedded on marketing pages for consulting support for my open source project that now I need to pay a dev or dedicate time to update. Or any other scenario where o have customers and potential customers that discover or interact with me at least partly through my github account.
Obviously OP understands the company's decision for changing his username but he was allowed to have it in the beginning. It is odd to backtrack and force a name change without notice. Github should update their policy with new reasonings for allowing themselves the ability to change your username(or account details) without notice. Afterall If Github can change my username(or account details), I would like that to be communicated to me.