You're right, Garmin wouldn't move the City Navigator. It's not listed in the Basecamp MapInstall window, so it's not available to be moved to the Rino. Apparently it's locked to the Montana. It originally was purchased on a dvd which I have.
I'll take a look at the firmware downgrade option next. I saw numerous older firmware files at your link. I'll do some research as to what will work on this Rino device. I'm worried that I'll be SOL without any gps if the wrong firmware "bricks" this device. This ventures into uncharted territory for me.
I thought the Worldwide DEM Basemap had major roads, but that seems to be just a very vague topo presence when zoomed way out. I find that odd, maybe the roads are a part of City Navigator. I removed the NE Topo so the only maps on the unit are the Worldwide DEM Basemap, a Digital Globe and a Grand Canyon demo USGS Quad.
I don't find a gupdate.gcd file on the unit. There is a GarminDevice.xml file that has reference to the latest software update version.