The first time you start Pokémon GO, you are asked to put in your username. This has to be a unique name that no other user is currently using today.
Can a nickname be changed afterwards? It is already known to be easy to change a Pokémon’s nickname. But it is indeed also possible to change the username of your account as well. Niantic is the maker of the Pokémon GO game and they decided that it is indeed possible to change your nickname to something else.
However, there are limitations. Niantic will only allow to change your nickname if you used your real name as a nickname. To protect your own privacy, you can fill in this form in order to request a username change in Pokémon GO.
Enter your current nickname. You do not need to include a preferred new nickname with this request. If your request is accepted, you’ll be prompted to create a new nickname once the change is completed.
Remember that it can take a couple of days before they reply to your request.