https://www.polygon.com/2019/2/21/18235337/pokemon-go-change-teams-medallion

Niantic and The Pokémon Company will finally let Pokémon Go players change their team allegiance in a forthcoming update to the mobile game. Starting Feb. 26, players will be able to purchase a new item called a Team Medallion, allowing them to switch to Team Mystic, Team Valor, or Team Instinct.

A Team Medallion will cost 1,000 PokéCoins when it goes on sale later this month. Players can purchase PokéCoins from Pokémon Go’s in-game shop with real money, but they can also be earned through play.