The main ProofBuddy plugin is free. You can use it as long as you want on your site. The same is true of some of the plugins.
There are several paid plugins in the works, and the plan is for those to have upgrades included as part of the price. WordPress makes the upgrade process so easy that whenever we push out an update, you will see a notice in your administration pages and be prompted to upgrade.
That said, that’s the plan for now. It could change in the future. No plans to, but we do need to leave that option open just in case.