It's easier than most people give it credit for. A lot of the complaints are from attempts to loosen the building code. There's savings of many millions on the table per refit if they manage to pass those, but they're not as needed as people say. For instance you lay down a raised floor to run utilities, and you can push sewer away from the core for relatively cheap and without shared bath/kitchen.

That being said, a return to allowing boarding house style housing would also not be the worst thing in the world for some buildings, and would probably do a lot to reduce homelessness. Hell, if I were still in my early 20s I'd be into the idea of a room to rent with shared bath/kitchen to save some money even not necessarily requiring the reduced in unit amenities.