I was wondering if there was some way I could go into the site's code and directly edit the variable for my username? Or anything like that?
In short no. However there seems to be a hack out there (I've not tested this)
As per Facebook’s condition and terms, it does not allow the users to change their facebook usernameafter setting one. But this simple technic allows to do that. To change your username, you just have to deactivate your account. But don’t worry you will not loss any data, because you can re-activate on next log in. Follow the steps below.
Step 1 Go to account settings from the button on top right Click on ‘Security’ Then click the option ‘Deactivate your account.’ Give explanation as “Its temporary”.
Step 2 Then log in again with your email and password. Done Your account is re-activated.
Step 3 Go to account settings from the button on top right. Edit username. Done. Your account username is changed.
I don't think you need to deactivate your account to change User Name. See the following blogspot how to do it: Change Facebook Login ID
I have changed my Facebook ID without any problems.