You can restore your previous purchase and continue to use the Pro version for free when you use the same OS on the new device. For example, when you change your device from an iPhone to another iPhone(or any other Apple device) or from one with Android to another with Android.
Please check how to restore purchase on the following link.
👉I paid for Pro, but why can't I use it? (Restore purchases)
However, when your new device does not use the same OS as your previous device, for example, in cases you change your device from iPhone to a device with Android or a device with Android to iPhone, it is not possible to restore purchase.
Even though when you already purchased the ImgPlay Pro on your iPhone, another purchase is required to use ImgPlay pro on Android devices.
Because iPhone and Android phones have different systems(OS) and different stores to purchase applications. Also, user accounts are different as well. Therefore, it is not possible to restore the purchase. To restore purchase and continue to the pro version for free, you will need to use the same account in the same store.
Thank you so much!