What is your upgrade policy?
If you have bought and/or installed an Android app, upgrades are managed through the Google Play Store and are made available for free.
If you have bought and/or installed a Windows application, minor version upgrades (eg 2.3 to 2.4) are also normally free. Major version (eg 2.x to 3.x) upgrades may require a modest upgrade fee.
If you subscribe to notifications from the relevant program page on this website you'll get the occasional email notifying you when an upgrade is available.