People online have been reporting that Nintendo of America is offering to trade in old versions of the Nintendo Switch for the newer versions that have a longer battery life – so long as they were bought on July 17, 2019 or after (ie. the launch of the new Switch). [Thanks, Reddit!]
According to those who have tried out the service, Nintendo of America will ask you to pack in the Switch unit as well as everything else that came with the box, but not the original box itself. A proof of purchase is also required. Shipping costs and insurance are paid by the user. It seems that so far, only Nintendo of America is doing this service.
That said, save data will not transfer, nor are Joy-Cons. For the latter bit, Nintendo will ask whether you prefer grey or red/blue Joy-Cons with the new model instead. It seems that the entire process will have you wait around a week and a half or so.
Finally, Nintendo has confirmed that the reason for the longer battery life is not an improved battery, but rather a processor upgrade, which is why it seems to also have a slight GPU increase.