Last month Apple made a recall notice public for the first generation iPod nano players. Apparently, there was an issue with the entire generation that could lead to overheating. Apple will be offering all owners a replacement unit. That would seemingly hint that the first gen units will be replaced with the new sixth generation units.
It's time to start digging around in the drawers around your house and find those old units for a free upgrade. The recall was only on the first gen nano, all other versions of the nano are the same. Users have previously reported long delays getting their replacement nanos. New updates are suggesting the hardware is the latest nano model.
It would be hard to replace all first gen units with other first gen units. Apparently, Apple did have some refurbished units lying around. Perhaps the plan was to get the recalled units in and fix them for an exchange supply. Serial numbers on shipping replacements resolve to the newest models.