Can you change teams?

Solution 1:

As of February 26, 2019, the feature to switch teams has been implemented. You can switch teams by purchasing the new "Team Medallion" item from the shop.

While the ability to switch teams is now available, there are a few things that need to be considered:

• Interested Trainers will be able to purchase a Team Medallion for 1,000 PokéCoins from the shop.

• Choose your team wisely, Trainers, as you may only purchase a Team Medallion once every 365 days.

• Any of your Pokémon that are in Gyms when you change teams will be returned with no PokéCoins earned once it’s defeated. You also can’t battle your own Pokémon, so if you return to a Gym one of your Pokémon is defending, your Pokémon will be returned to you with no PokéCoins earned

Solution 2:

As it stands, no. The team you select is permanent, however, you can make a new account and request your old one to be deleted (available on their website) if you'd like to switch teams, at the cost of all your progress.