I don't know much about how much changes from version to version... I've only recently started with oxwall... but best bet.. contact the plugin developer. Chances are they've looked at how much work (if any) is needed.

This is developer's responsibility to define his plugin's update policy. Oxwall platform just provides a mechanism to autoupdate plugins from store. By default, the most developers provide updates for free. But to avoid any confusion, contact them directly before purchase, 

So I  suggest the store establish a specification to make dvelopers declare the policy clearly if the plugin is for sale.