Are there any regulations preventing one from converting the cargo version of the Airbus A380 for passenger use?
Converting a freighter to an airliner would require lots of work:
- you have to install an HVAC system capable of supporting hundreds of passengers, which means installing air ducts from the engines to the cabin, etc. Major surgery to the wing and fuselage.
- you have to install a sufficient number of doors and emergency exits (with associated systems). Major surgery to the fuselage.
- big changes to the electrical system to support e.g. entertainment for the passengers.
So the conversion would require taking most of the aircraft apart.
There is no cargo version of the A380, but for other types:
There are no regulations prohibiting converting a cargo aircraft to a passenger one. It will just need to meet all of the passenger certification requirements, such as evacuation times, seat and structure crashworthiness, oxygen systems, and so on.
I don't think this has ever been done because it is extraordinarily expensive. It is cheaper to convert a passenger aircraft to cargo because for the most part you are removing items instead of installing them. Passengers also prefer new aircraft, so after going through the cost of converting the aircraft you'll struggle to fill it with people.
Can cameras replace windows? From a legal standpoint I'm not sure, I think yes as I recall a patent for it, but that would be an interesting question to ask here.