We’re sure that we are able to retain the apps that are installed in Windows 8.1, Android and IOS when doing the In-Place Upgrade but what about the one for Windows Phones and migration? With the Start Screen Backup feature for Windows 8.1 and Windows Phone 8.1, you should be able to synchronize the Start Screen layout placements as well as the App Data between devices. I don’t know if the IOS and Android can do the same thing but for the App-related stores, apps you have purchased from will be kept in your account that you can choose to download again but it can be annoying when trying to re-download those apps again for your new device. Why isn’t there the App migration method for IOS? The Android surely has one due to the use of external storage but not all apps can be compatible depending on your Android device.
Right now, there is still the Developer Preview version of Windows Phone 8.1 and and when the OS is out in April 14, users are able to download the update to update the OS to the final version.