Only the ip address of the nameserver may be changed. If the ip address is already in use by another record, it will fail, you will be notified by email. In this case, just use the nameserver which has been already using the desired ip address.
To change the name, simply create a new nameserver record. If the name or the ip number of this nameserver record is already in use by another one, it will fail; you will get an error message by mail.