Upgrading from the free edition is very easy. Pro works with the data structures from the free edition and adds additional data when needed.
Simply install the pro edition and make sure to uninstall the free edition, as there will be two menu entries otherwise.
Downgrading from the Pro edition is possible but highly discouraged as the additional data added by the pro edition will be lost when editing the story map with the free app, this includes but is not limited to settings, timeframe information and virtual issue links.
The free edition will also not work for story maps where a custom epic type is configured.