@TeamYouTube I'm looking for help in custom URLs and Legacy URL's but have not found an email address for support. Can you connect me to a support representative?
YouTube treats legacy username URLs as your primary channel URL and they can't be changed. However, please send a feedback report with these steps: https://goo.gl/WdI0BK . You may not get a direct response, but we review everything and use the details to investigate. Thanks.