I' d like to know it too.
How can change Username to another name without new registration

Lokht......Rocky told you how in post 4......you can do it once. Anything beyond that Staff gets suspicious.

Go to Settings (cog icon) -> Profile and there should be a Change link next to your username.

You can only change your username one time, I believe. After that you would need to write support:
https://support.chess.com/customer/portal/emails/new
Please help