Maybe restart/refresh the app when people buy the ad-free version?
I just bought it, and after a pop-up that says "Enjoy the ad-free version" or something, I was immediately brought back to my feed with an ad on it.
I get why it happened, and in my case it was just a funny moment that prompted me to restart the app, but a non-tech-savvy person might find it disturbing and waste your time with a support request.
"WTF, I bought it and my app restarted, now I lost my position in the feed! Is this how you treat your paying customers?!?!!@#$!@#!" <-- user, after such a change, 98% likely