The Team Medallion is an item in Pokémon GO to be introduced on February 26, 2019 [1]. It's a single-use item that allows a player to change teams. It's available in the shop for 1,000 and can only be purchased once every 365 days.
A unique coin that enables a Trainer to change teams. A Team Medallion can only be purchased from the shop once per 365 days. Confirmation: You will not be able to purchase another Team Medallion for another 365 days. Your Pokémon currently defending Gyms won’t earn PokéCoins when they’re defeated.