My former police service (well, actually the government ministry that does these things), gutting a 1920s building to 'maintain the aesthetic'. It does look nice but they figured it cost about twice as much as had they simply torn the original building down. The community collage right next door build a couple of dorms in a style that reflected the old buildings. They look surprisingly the same.
Many of the RHUs I have seen on a few bases look to be Cold War-era 1 1/2 storey rectangular boxes. If the thought was to go engineered and plop a new house on the existing foundation, It would probably have to be a custom design since I doubt any builder of engineered homes makes something that would fit that footprint. There might be some savings if the design was repeatable enough. Are most of these places slab-on-grade or do they have basements, or perhaps 'it depends'?