Once you've downloaded the update (when it's available later today), you'll be able to head to Wedgehurst Station and meet one of two new trainers who are coming in the Expansion Passes. Pokémon Sword, trainers will meet Klara at the station, while Pokémon Shield people with find Avery waiting for them.
Perhaps more excitingly for Gen I Pokémon fans, you'll also come across a Galarian Slowpoke which has somehow appeared outside its usual habitat located in the Isle of Armor, the first of the two upcoming Expansion Pass areas. You'll be able to catch it and once you find certain items from the Isle of Armor (for Galarian Slowbro) or from the Crown Tundra (for Galarian Slowking) you'll be able to evolve the Pocket Monster.
This freebie Pokémon is just one of over 200 returning monsters coming as part of the two expansions, greatly increasing the Galar Pokédex's current roster of 400 monsters. Players without the expansion will be able to obtain those monsters (after an update) through trades if they don't fancy paying £26.99 / $29.99 for the extra content.
The update isn't live at the time of writing, but Nintendo says it's coming today, so keep an eye out for it on your Switch.