No , you do not have to purchase the latest version and you can use the PaperOffice you have purchased for a lifetime at no additional cost.

Please note that the updates contained in PaperOffice Plus are only ever delivered for the current version (release). This means that you will NOT receive any updates for an outdated version.

Please also note that database compatibility is only ever guaranteed for the previous version, e.g. an update from version 2019 to version 2020 would be possible because the database structure is automatically adapted to the changes.
With a cross update (e.g. 2015 and 2019) an automated data transfer is not guaranteed. In such a case, contact support to consider manual data migration (fee-based service).