The bestbuy $300 deal works in store, even if you don't see your exact model number online, for your trade-in watch. I went this route since some of the trade-in S9 45mm watch trade-in figures were $180 instead of $300. I didn't want any surprises later after I got the new watch.
So I went in with my box, cable, band, and watch. They only wanted the watch, so I got to keep everything else. They entered my serial number and said the trade-in would indeed be $300. At that point I turned off Find MY, and reset the watch. Once they could see on their system that the watch was off my account, they issued a gift card.
When you place your order for a new watch, they charge tax on the price of the new watch, not the watch after the trade-in discount. This is different from how Apple does it, so that cost me about $22 more than I expected. Still, $300 is a nice trade-in, especially since I paid $387 last year, and I get to keep the old band.