https://gizmodo.com/nintendo-is-a…1837315161
Basically: what the headline says. If you bought the first-gen switch after July 17, Nintendo will replace it with the new model for just the cost of sending your "old" one back to them.
There doesn’t appear to be a web form or anything, but you can just call their North American service number to initiate the exchange. 1 (800) 255-3700