Google first unveiled Gmail to the public on April 1, 2004. Now, 22 years later, Google is finally letting some Gmail users change their account's username while retaining everything else in their account. The ability for Gmail users to change their username was first teased by Google late last year. And now, as of Tuesday, every Gmail user in the U.S. can officially change their username — that's the part that comes before the "@gmail.com" – to whatever they want, as long as the new username is available. Aside from the username, everything else with the account remains the same. All emails and files associated with the old username will continue to exist in the account for the new username. What happens to your old Gmail address? Google says it will retain that username for the user so that emails sent to that old address continue to arrive to the new username's inbox.  So, are you ready to change that Gmail username you created while you were still in high school? Here's how to do it.…