No offense to the OP but the customer reaction seems obvious. I've thought about doing this with my apps before and then thought about how I would feel as a paying customer if the developer suddenly thrust ads upon me and asked me to pay to remove them again (or didn't give that option). If you want to put ads in an app people have paid for the only real solution is to release a separate "lite" version with ads. It might not seem like the best decision for your business but it's better than pissing off the people that cared enough to pay for your app in the first place.