In the past, Pokémon games were usually superseded by an enhanced version (or versions), which added new features to the original game. For example, after Pokémon Sun and Moon came out, Pokémon Ultra Sun and Ultra Moon were released, with a new storyline related to the Legendary Pokémon Necrozma, new Pokémon forms and Ultra Beasts, plus a meaty post-game mission.
For this generation, Nintendo and The Pokemon Company will be releasing a paid Expansion Pass for Sword and Shield instead. The expansion pass will be divided into two sets of content.
The Isle of Armor: This will be released before the end of June 2020.
The Crown Tundra: This will be released during Autumn 2020.
Both of these will feature brand new environments, new Pokémon, additional story, new gameplay features, as well as 100+ returning Pokémon each (for a total of 200+). Once the content is released, you can use your current save data to play it. To access the new content, you must have reached the Wild Area, which occurs within the first few hours of the game.