[heckanoobs]

I think you can get by with adding timestamp support to a single feature flag. So the code has IS_PREMIUM everywhere but when you add a new use of it you also manually record the time. Since you know the expiration time of their license you can always determine what features are unlocked based on that.

Then you just develop the app as any other demo-with-upsell app. Users even continue to get bug fixes and other updates after they let the license lapse.

In cases where features are removed, combined, refactored, you'll have to be judicious about what timestamp you apply to the refactor. I would resolve in the customer's favor personally.

Edit: Thinking on this more you might also want a rule like feature flags are only implemented at the view layer. That way your biz logic never gets tripped up on flag checks as features cross paths under the sheets. So its does require a fair amount of code discipline to pull this off