Changing Username

I'm having a lot of trouble changing my username. I used my Facebook account to first create my account years ago. Now that I'm getting more into Musescore, I want to change my username to something more original ("Piano_Warrior"). However, when I go to change it in "Account" under the "Settings" tab, it requires me to enter a password to make changes to my email or username. But I don't HAVE a musescore password since I log in through Facebook.
I researched some more and tried the idea of merging accounts. I made a new account with a different email titled Piano_Warrior, but when I went to "Merging Accounts", it didn't accept my username entry for the old account, "Parker Beus", saying it was invalid. Isn't my old account's username still Parker Beus?
Please help me have an account titled "Piano Warrior" with all of my old scores attached. I don't want to lose what few followers I already have.