Keitaro is correct. SOLAS 2010 prohibits the use of some materials used in older ships (e.g. combustible ones), and it would also be necessary to make structural changes to many older ships. As an example, all stairtowers on the "Norway" would be required to cover all decks. Right now, they don't - mainly due to the ship's former two-class design. Also, many passenger hallways have small "side hallways" which are also a problem for SOLAS 2010.
Yet, it would be possible to convert the "Norway". But that would only make sense if she also gets new Diesel engines. And on the other hand, if she is to get new Diesels, that only makes sense if she's able to sail for many more years to come.