No, your Username and Password will continue to function as normal in ESET products that use these credentials.

If I convert my Username and Password to a License Key, do I need to upgrade my ESET products? Converting your credentials will not affect your existing ESET products-both your Username/Password and License Key will remain valid. If you recently upgraded to version 6, or do not have your License Key, you can use ESET License Administrator to convert your existing Username and Password to a License Key. Version 6 business products use a License Key. Version 5 and earlier ESET business products use an ESET-issued Username and Password for activation.