To start with,for many conversions you might need changes to the exterior walls to be able to pack in enough flats with required access to windows, depending on your local regulations, so it's not a given even the exterior walls would be untouched.

It certainly can be done - given many have been done, but it's not at all a given it'll be cheaper for all properties vs. tearing it down (which would still keep the utilities to the property, of course) and starting over.