If you have save game data for Pokémon Sword and Shield, and Let’s Go Pikachu and Eevee you’re in for some bonus content. As posted on the Pokémon Legends: Arceus website, if you have Sword and Shield save data, you’ll be given a Land Forme Shaymin.
The research request will be added to the game after the end credits, so you will need to wait until post game to catch Shaymin. You also will be given a Shaymin kimono set. You’ll be able to claim it after you join the Galaxy Expedition Team in Legends: Arceus.
Finally, you’ll be given a Pikachu or Eevee mask to wear if you have Let’s Go Pikachu and Eevee save data. The website says you will be able to claim both masks in the game.
Nintendo shared details on the day one patch coming to Brilliant Diamond and Shining Pearl, and you can view it by clicking here. If you are unsure how to update the data for the games, they made a support article helping anybody out.
Edited by bobandbill and Rabinov.