So when it comes to copyright music can actually be divided into two things: the song itself (the lyrics and melodies) and the performance of a song (the recording). I don't know the entire situation but I'm guessing Taylor Swift owns the rights of the songs themselves, but not the rights to the recordings of the songs that were made, so she's in her right to make her own recordings of the songs and then she would have full ownership of those. (Sidenote: I study composing for media and though copyright is part of my study, I'm not an expert)