You can keep using ImgPlay Pro even after changing your iOS device by restoring your purchase.
To learn more about it, please check the following guide.
👉I paid for Pro, but why can't I use it? (Restore purchases)
However, if you change your device from iPhone to Android, or Android to iPhone, purchase restoration is not possible. This is because iPhone Android have different systems(OS) and app stores. Also, user accounts of each platforms are different. To restore purchase, you need to use the same account in the same store.