Considered charging less for the full product (say $10), and simply having multiple versions available as you post major new releases?
That way, users always have a way to downgrade to previous versions as needed, you probably get more customers due to the entry price, and everyone just understands that the latest version is available for $10 more.
That seems to be the whole app store model: lower prices, more sales -- no "upgrades" in the classic sense. And, with a low enough entry price, I think users would be accepting.
If they don't buy the latest, as a developer you're no worse off than you are today (where they get the latest for free).
That way, users always have a way to downgrade to previous versions as needed, you probably get more customers due to the entry price, and everyone just understands that the latest version is available for $10 more.
That seems to be the whole app store model: lower prices, more sales -- no "upgrades" in the classic sense. And, with a low enough entry price, I think users would be accepting.
If they don't buy the latest, as a developer you're no worse off than you are today (where they get the latest for free).