Or use "The Dutch Model":

Pay for the major version, get all of its updates. Then pay for update (to next major plus its updates) with a discount.

If you don't prefer the pay, you can keep what you have.

This is what Reaper, Forklift, CameraBag and countless others do, and it works very well.

Edit: This comment contained Forklift as an example before, but they have changed their model, so it's removed.