PKHeX, a Pokemon save file editor for all mainline Pokemon games, is now compatible with Pokemon Sword and Shield.
This just comes one day after the games were launched worldwide, thanks to the work of those at Project Pokemon. As Pokemon Sword and Shield save files could now be modified, you should be extra cautious about the Pokemon you receive in online trades as they could have been modified through PKHeX. Other aspects of the game could also be modified, such as the number of items or Pokemon a player has.
For more details, please check out the Project Pokemon website.