It is a free upgrade to all users who purchased a license or upgrade after 1 March 2017. Users of older versions will need to pay to upgrade to help cover our development costs (e-mail us if you are unsure when you purchased).
You don't need your old serial number or installation to upgrade (we'll send you everything).
What version of MailList King do you own? Your version is displayed in the About Box (Help > About on the menu)