You can’t change the rules after the purchase, period. If notability wants to go with the monthly payment option it’s their right, but you can’t take back what you’ve already sold. Not even adobe went that far.While it's of course sad for any user, I'm really hard sympathizing with a comment like this:
So he's been using the app for 6 years after paying 9 bucks and some more for the macOS app. And it's supported him through three degrees. And how he's upset the company who's obviously provided a great product want a stable financial model to help them keep providing a good app. Do they all think these companies can really survive for almost a decade on a $9 purchase and free upgrades after that.
The grandfathering is that they don't have to pay for the first year. I guess you can argue that's not enough, maybe add more time. But still...
I hope they reconsider.