The email and other data in your account remain untouched when you migrate to a new username, and you will continue to receive mail destined for your original address. You can also log in to your account using either username. Some Google products and third-party services that access Google data will continue to display your original email, but the new username becomes your primary Google account identifier. If you don’t want to see that old handle anywhere, the only option is still to create a totally new account.
Change your Google Account username in a few simple steps
Google also notes that the old username remains attached to your account as an alternate, and you can switch back to it at any time. It’s unclear if this is a one-time process or if you can switch back and forth at will. Likewise, Google has not explained what happens if you end up with multiple alternate addresses over time. Google says you can’t delete your new addresses, so it’s possible you could have a whole gaggle of Gmail aliases.
While the process should be smooth for most users, the complexity of Google’s account ecosystem means there are some caveats. If you use a Chromebook, for example, changing your username requires you to log out and back in to the machine. Google also recommends backing up your local files first. Chrome Remote Desktop connections will stop working, too, forcing you to delete connected machines and add them again under your new address. Most other issues should be short-lived and will clear up after a few hours or a device restart.