Thanks for asking about this here!
The username of an existing Spotify account cannot be changed. If you decide to create a new account, a username combining numbers and letters will be created automatically. It will be visible only to you. This is happening due to security reasons.
You will also get a display name. This is what all users will see when they come across your account. You can choose a display name in the 'What should we call you?' field during sign up.
As to your music - you won't lose it. If you wish to have a different username, you must create a new account and you can transfer all of your music to it. You can read more about it here.
Last but not least - it's very important that you cancel your subscription on your previous account and subscribe to Premium on the new account. If you have subscribed to Spotify through a partner of ours, you must reach out to them so they can fix this for you.
We hope this info helps.
Keep us posted!