During the summer I heard a KQED segment of some prominent examples of this from SF and why it's very expensive to do, rare to be able to pull off. Maybe it was this one: https://www.kqed.org/forum/2010101889483/what-would-it-take-...

Just to pick one example out of a hat, the discussion of elevators was one that made an impression on me. They said the allocation of elevators for residential vs office space is very different, and in some SF examples they had to put in entirely new elevators and staircases.

Seems there's lots of these things you wouldn't think about unless you're in that industry...