Hi,
you can continue to use the version you originally purchased for as long as you like.
The keys always only work for the main version they were purchased for, this wasn't changed (e.g. you can't and never could use a PDF Architect 3 key to activate PDF Architect 5).
Beginning with PDF Architect 6, there is now also a subscription based model which lets you always use the latest version. The subscription comes with additional benefits like being able to use PDF Architect on up to 3 machines, which many users asked for. Previously you needed to purchase a separate license for every machine.
You can however still purchase a perpetual license which allows you to keep using PDF Architect 6 as long as you like (but can't be used for future versions): https://www.pdfforge.org/pdfarchitect/personal/perpetual
Best regards
Robin