Convert existing iOS paid app to freemium model with in-app purchase

There is now an Apple-approved way to do this on both iOS and macOS. The originally downloaded version of the app can be obtained from the receipt using the info key Original Purchased Version. You can then decide whether to unlock features if that version predates the switch to IAP.

For instance, once you have retrieved the receipt info:

NSArray *versionsSoldWithoutIAP = @[@"1.0", @"1.1", @"1.2", @"1.3"];NSString *originalPurchasedVersion = [receiptInfoDict objectForKey:@"Original Purchased Version"];for (NSString *version in versionsSoldWithoutIAP) {    if ([version isEqualToString:originalPurchasedVersion]) {        // user paid for the currently installed version    }}

For more info see the WWDC 13 video Using Receipts to Protect Your Digital Sales. At 5:40 the presenter comments: "I think the most exciting thing that's in the receipt this year, especially for you guys if you have a paid app in the store is that we've included information in the receipt that's going to let you do a transition from being a paid app to being a free app with in-app purchases without leaving behind all the customers that have already paid for your app."

With iOS7, iOS app can verify app store receipt, which contains app download date.By using this donwload date, you could determine if a customer is previously purchased or not

First, I just want to say I personally think the freemium model is great. It has worked out very well for many developers. People love to download free apps, and will do it on a whim, but pay much more attention to an app before spending $0.99 (Which is due to the effect of free - for more info on that, check out Dan Ariely's book Predictably Irrational)

For more info on freemium, google it - There have been tons of articles written about the success of it.

Ok, back to the actual question:

Theres a couple ways you can handle a situtation like this, although the unfortunate matter here is none of them are fool proof.

• The best solution would probably be for your users to have accounts. Without knowing the specifics of your app, I can't say whether or not user accounts are appropiate for your app. User accounts stored on your server have many additional benefits, including user management, and tracking what purchases a user has made. This will allow users who delete the app, and then re-install it, or get a new device, to maintain their purchased content. Furher, whenever you use in-app purchase, you should validate the purchase on your own server (or with Apple), which a server based user manegment system can all do. If your totally in over your head with creating your own user management server, check out Parse. Its dead simple to create an amazing backend server (for basically free)
• iCloud Key/Value type of system. I'm not very familiar with how this would work - so I'll move on.

• Another, not nearly as fool proof solution (but much quicker/easier to implement) is to use NSUserDefaults. You can store an object when the user makes a purchase, or with the date a user installs your app. Then if you issue an update converting your app to freemium. Then in the new update, check which purchases the user has made or the date they installed it, and react accordingly. For info on how to do that with NSUserDefaults, check out my answer to another question on implementing that: NSUserDefaults and app versions.But this solution does present the following pitfalls:

• If the user deletes your app, the NSUserDefaults are lost forever

• If the user didn't install the update setting up the NSUserDefault system, but then installed the update with the new freemium model, the app would treat them as if they hadn't purchased the content.

In summery, this is a difficult question, with not a lot of easy/perfect options.

Anyway,

Hope that helped!