Move Purchased App from Windows Phone to iPhone My Windows Phone got stolen and I decided to get an iPhone 4s as a replacement.
I bought the Threema-App for WP 8.1.
Is there any possibility to get Threema for iOS 9.2.1 without having to buy it again?
I assume you know that the application on the Windows phone can't, itself, be installed on your new iPhone. 

As to whether or not the manufacturer will allow you to "sidegrade" or not is a question only they can answer