https://www.cnet.com/news/pokemon-go-names-harry-potter-wizards-niantic/

While there's no release date for the game just yet, the opportunity to reserve an existing name is open from now until April 30. However, the game's creators Niantic point out this isn't a chance to create an entirely new user handle. You have to wait until Wizards Unite launches in your area to do that.

To carry your existing name over to the Wizarding World, magic your way to Niantic's reservation portal. You'll be asked to log into your Ingress or Pokemon Go account, and you can then reserve your name for Wizards Unite. If you have the same user name for both Pokemon Go and Ingress, you only need to reserve your handle once.