It's not a problem to change your username, but you can do it only once (as well as with page's username).
The name needs to be unique. I don't think you can change it. However, see if this response resolved your problem: first you need to go to the option deactivate account after that you need to choose an option then you say your account was hacked after you click on that you see something you can click on its say secure your account then click on here then click on here and then just follow the steps it will show you email or someone else and then go on go on and then on the end your username will be reset and then when you go to the option of changing your username it will wor. btw, I just did and it worked