A few point on the original question.
The pressuredifferential between inside and outside the cabin is the main item. the aircon/pressurisation system will be able to maintain a reasonable (FAR/JAR certifiable that is) pressure diff. and the hull and windows can be designed to cope with it. Once an aircraft model has been designed for a certain pressure diff. it is not easy to increase it so you are faced with what the pressurisation system can do after modifications and also what the structure (incl. windows and doors) can stand.That's for the cabin area. As a large part of your fuel/hydraulics etc. systems run outside the pressure hull. this must also be able to cope with the increased pressure diff. (now greater from within the pipes/boxes to the outside of them).
Then also the engines as said before. A lot more puff needed in thinner air.
The whole lot then needs to be re-certified. Lots of work for the manufactutrer and the authorities.
In the end... Is it worth it is some case? Need a good market to get your money back. the Airlines will often not accept to pay much for it.
A is dum (Is A?)