Ok, but some extra plumbing (and whatever sorts of engineering studies referred to) and electrical work surely can't as expensive as demolishing and rebuilding a whole building.
If it was just the plumbing, then maybe. But its not just the plumbing. Its the plumbing, the electrical, the AC/ventilation, fire codes, and so much more.
Not saying it can't ever be done, it really depends on the building. But its not necessarily a good assumption it can be done well in a cost-effective fashion.
These seem like extremely solve-able problems.