Can't be done unfortunately.
The Mini used a micro drive for it's storage, which is pretty much plug and play. You can pull it out without having to do any serious work other than opening the case.
The Nano on the other hand has the flash chip soldered directly to the main board. And this isn't just one or two solder connections, its a full chip soldered to the main board so for all intents and purposes it's not even possible to do it by hand.