There have been a number of questions related to changing your username in Pokémon Go, but I was not able to find one regarding a situation I had encountered.
A friend of mine recently starting playing Pokémon Go and made it level 5. After they selected a team, they weren't fully content with their selection and wanted to switch to a different team. Since selecting a team becomes available at an early level, starting a new account and choosing a different team would not take very long. However, their username was unique and personal to them and once a username is chosen, it cannot be used by another account.
In a previous update, the ability to change your username was implemented, but you could only change it once. Seeing how the teams (currently) do not provide any special in-game bonuses, they would rather stick with their username than switch teams.
So, if you change your username in Pokémon Go, does that username become available to another account or is that username still unavailable?