Due to a technical issue, existing Trainers may incorrectly be presented with the new player tutorial in some rare cases. If players continue through the tutorial, they will be prompted to change their Trainer nickname. This has led some Trainers to inadvertently change their nicknames when attempting to bypass this tutorial. We aim to resolve the issue in a future release of the Pokémon GO app.
If you experience this issue, please force-close the app and reopen it.