Unfortunately you are unable to change the name servers of domains you have with Cloudflare Registrar. The name servers will always be set to the ones assigned to your account.
If you wish to use alternative Cloudflare name servers, then the domain would need to be transferred to the account those name servers are assigned to, which may require transferring away from Cloudflare then back into the correct Cloudflare account, unless some kind of Internal Transfer is possible.
