The stewards will decline any request for username changes or usurpation on wikis with active local bureaucrats. You can get the list of local bureaucrats by going to the wiki in question and typing in the search box Special:ListUsers/bureaucrat.
For a list of local pages in the different Wikimedia projects where renaming/usurpations can be requested locally, please see the index of pages where renaming can be requested. If a project does not have any dedicated page, contact a bureaucrat via talk page and/or email and make your request.
Stewards might do renames on wikis with local bureaucrats if they are inactive for a long time. Please try to contact them first before posting here.