One-time purchases do not expire, but the six-month free upgrade period does. The release that you bought will continue to function as always.
Many releases are for maintenance purposes - bug fixes, or perhaps a new feature. Occasionally you may be proded for an upgrade, but there's no mandate. If your version is doing the job for you, no need to change. You can test drive the latest for a trial period, even if unregistered. If nothing catches your fancy reinstall your purchased version.
There is also a subscription plan, as noted above. You cannot order a VP version that's old, AFAIK, but you can download them from here.