I already asked this at the dealer already. You can't because the hardware doesn't support the software update. The screen resolutions are different, so the current screen won't support the software. The new navigation uses more updated hardware, so it's not something as simple as updating the harddrive software and calling it a day. You need the hardware too.
The new navigation is nice but not that big of a difference that would make me kill over to get one. Just updated graphics/map, bluetooth streaming, zagat, weather updates, some revised voice recognition, and that's pretty much it. I've fiddled with it already and the menu layout and everything is pretty similar.