I'm pretty sure you'd have to replace the whole radio. May end up being rather cost prohibitive in the end.
I've got the 430N in my GC, but truthfully I'm much happier with my stand alone Garmin GPS. With a standalone unit, you're limited to only being able to enter information when you're stationary, not to mention map updates are a fraction of what they are for factory units. Hell they're free if you get a model with lifetime maps and traffic.
Another thing I'm not super thrilled with related to the factory unit is you cannot read anything on the LCD if you're wearing polarized sunglasses because of the angle with which the LCD installed. I don't have that issue with my stand alone Garmin.
ick yourself up a stand along garmin unit, you'll be happier.