<DISCLAIMER>
This is a concept! I don't know if it is possible to do this!
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My Concept:
1. Take the update file out of the iOS Device
2. Get an update file for another phone (In this concept, i'll use the Android)
3. Extract files from both the iOS and Android update file.
4. Replace all of the iOS Files with Android (BUT DON'T CHANGE THE SIGNING!)
5. Recompile the iOS Update file.
6. Use it (or distribute it)
This might work because it will change EVERYTHING on the iPhone, but since the signing remained the same, your system will register it as a legitimate iOS Version, so it will be allowed.
Please tell me if this will work!
Read more: Any Phone OS on iPhone! (Concept)?