This morning I tweeted that there's no way to convert a Spotify account from Facebook login to email and password login. I even spoke to Spotify support who said:
As your account was created via Facebook, it can't be unlinked. Don't worry, we can free up your email address so you can create a new account. Once that's done, we'll transfer all of your tunes and student details across.
I complained about this in one the Slack groups I'm in, and Nathaniel suggested I try doing a "forgot password" to set a password on the account and it worked. After that I was able to disconnect Spotify from the Facebook settings page.
The above steps that I followed have also been confirmed to work by Wade.