I haven't run the numbers on basic water, sewer, and electricity needs for residential vs office per square foot, but I would not be surprised if they were different.

In principle you can strip a building down to the bare walls and floors, and redo all the internal systems, but at some point it becomes cheaper to demolish and start over.

There are mixed-use towers in come cities. Trump Tower in Manhattan is an infamous example. Street level +/- 1 or 2 floors is retail, then office space above that, then apartments on top. But it was planned and built that way from the start.