wp-config.php file updating

I wish to upgrade my word press 2.51 installation to 2.6 but I am not clear about whether in the wp-config.php file the SECRET_KEY phrase needs to be replaced or kept. The 2.6 wp-config sample file has:
‘AUTH_KEY’,
‘SECURE_AUTH_KEY’,
and ‘LOGGED_IN_KEY’,
Do all of these need to be new or should any of them have the existing (2.51) SECRET_KEY phrase substituted or should the SECRET_KEY from 2.51 be additional to these three?