Typically you'll have to put in all new electrical and plumbing, because the existing office level capacity is no where near sufficient. You'll have to completely redesign HVAC and ventilation as well since you can't have an entire floor be one climate and you'll need exterior ventilation for showers/cooking/laundry that aren't a consideration in an office. There might not even be enough space between floors to accommodate those things.

Then there's other code considerations like windows and fire exit distance requirements that become harder to meet once you start adding walls.

If you have to move all the floors, walls, utilities, elevators, and exits, how much of the original building are you actually leaving intact? Often times, even after doing all that, a conversion still isn't viable because there's just too much floor area and not enough windows to make any layout that isn't a 10k sqft single room studio viable.