I'm not sure if you can, but that behavior was annoying enough to turn me off from buying the App Store version entirely. A gentle reminder is good enough, and

I'm not sure if you can, but that behavior was annoying enough to turn me off from buying the App Store version entirely. A gentle reminder is good enough, and might induce me to pay for the new paid version. However, if you take an app which used to be free, and then nag me constantly because you decided to make a non-free version of it one fine day is annoying.