I want to get my username back

So ı had a another account that ı closed, would it be possible if ı reopen my old account change my username to the old one and close the old account again?

Or you could just ask Support for permission to reopen the other account as your allowed second account.

Yeah but ı dont need another account right now , ı just want to have that username for this account, that account has like 500 elo and ı have 1600 elo rn so it would be useless and ı dont want to grind so that account has 1600 elo too, cant ı open the old account ,change the name of the old account , make the name of this account to the old one and close the old one again?

İf ı make my old acount my second account will that account also get a diamond membership?

No.

But you could just make it your second account and close it again after you're done

Yeah thats what ı want to do, ı guess ım just gonna ask the staff if it would work

If the email address is different, you should be able to reopen and change the name, assuming you haven't changed the name in the previous 90 days

https://support.chess.com/en/articles/10971891-how-do-i-reopen-my-closed-account

Not sure how it handles it if you're using the same email in this account