Let's see if I have this correct, you wish to move your facebook account from a free facebook/Spotify created account to a created Spotify Premium account, so as to link your facebook account to the Spotify premium account correct?
If this is the case you will need to first go to your facebook account and access the apps page, and then remove the Spotify app from your facebook account. Then you need to contact Spotify support via the contact form. And you will then be requested to confirm some detailed information of the account to Spotify customer support so as to have the facebook/Spotify account deleted, once the facebook/Spotify account has been confirmed as deleted, you should be able to then to add the Spotify app back to your facebook apps page, and then be able to link your facebook account to the Spotify premium account you wish to. Otherwise if you do not do this you will have two accounts in the system, and facebook is notoriously picking about dual Spotify accounts attached to facebook accounts.