Exteriors may stay the same, but the way we use rooms changes all the time:
"Real estate agents call this phenomenon 'churn,' and it's been around for a lot longer than historic preservation. Still, a landmark interior is required to meet the same stringent criteria as a landmark building. It must be 30 years old and operate in the public realm. To survive that long intact, it has to be lucky enough to have a sympathetic owner-or more likely, a string of sympathetic owners who have understood and appreciated the space. One false step on the march to 30 can leave it vulnerable to alterations and eventual extinction."