I'd rather prefer the demo version be published as an separate app, since you can't remove the advertising SDKs entirely even after you pay. Even if the developer was payed and stopped displaying ads, advertising companies have zero incentive to actually disable their code. It might still be spying on users in the background, and you can't be sure about it unless you go to great lengths to see what servers the app is communicating with. Worse, even if the SDK did actually stop functioning, it might be too late since you can't disable ads before first launch.