Any east coast city should have an easier time of it, because the main determining factor for retrofitting office buildings into residential is the distance between the center of the building and the exterior, in order to make sure there's enough windows in each unit. Any office building built before the advent of the electric light should be just fine, so there's plenty of opportunity in places like Philadelphia.
Where I worked in Longwood (Boston), the offices surrounding me were massive, historic buildings with plaques commemorating their origin. They were originally built for housing nurses and doctors who worked in the area's hospitals (there is a concentration of hospitals in Longwood).
So in this case, these were built and used as housing before a later conversion to office space.
These are currently utilized hospital buildings. IMO, they are unlikely to be converted back to residential at this time.