How can I change my character's appearance after starting Pokémon GO?

>At this time, it is not possible to change your avatar or your team.

Cited from here

Since version 0.31, you can re-customize the avatar from the Trainer profile screen (click the right bottom menu).

As of version 0.31.0 you can now change your avatar's appearance by navigating to the new menu in your profile and tapping the "customize" option.

Unfortunately after the initial customization there is currently no way to re-customize your character

Since Patch 0.31 this is no longer correct.